The report, released Tuesday morning by Stanford University’s Center for Research on Education Outcomes, provides perhaps the most thorough perspective available of the landscape of charter schooling, which has grown significantly in recent years.

Macke Raymond, CREDO’s founder and director, said that the report sketched a picture of continuous improvement for the charter sector over the last 15 years. The center’s first national analysis, issued in 2009, showed charters under-performing traditional schools in both core subjects; in a 2013 follow-up, they slightly bested traditional schools in English while still lagging in math. That movement represents a modest silver lining for American education, she said, after a prolonged period during which learning — as measured by standardized tests like the National Assessment of Educational Progress — largely stagnated even before the pandemic. 

“When you compare [our findings with] the results of NAEP — which, over an equivalent period, have completely flatlined — what you’re looking at is really the only story in U.S. education policy where we’ve been able to create a set of conditions such that schools actually do get better,” Raymond argued.

The new study focuses on charter school performance in 29 states, as well as Washington, D.C., and New York City, incorporating standardized test scores between 2015 and 2019. All told, over 80 percent of tested public school students were included in CREDO’s data set. More than 1.8 million charter students were each paired with a “virtual twin” (i.e., a nearby pupil possessing similar demographic traits and prior test scores) enrolled at the district school that the charter student otherwise would have attended.

The research team calculated that charter school students gained the equivalent of an additional 16 days of learning (based on a traditional 180-day school calendar) in English compared with similar kids at district schools. Their six-day edge in math was smaller, though still considered statistically significant.

But even those averages, comprising millions of student measurements across the country, contain significant variation. Black students attending charter schools gained 35 days of growth in reading and 29 days in math — as if they’d attended school for an extra 1.5 months over a single school year. Hispanics enjoyed 30 extra days of reading and 19 in math. By comparison, white and multiracial students lost the equivalent 24 days of annual math learning in charter schools. 

Smaller sub-groups experienced similar divergences. Poor students saw much higher gains in charters than in traditional public schools (23 extra days of reading growth, 17 extra days in math), as did English learners (six extra days of reading, eight in math); students with overlapping designations (such as both African American and low-income, or both Hispanic and English learner), also made considerable strides

By contrast, special education students were seriously stymied, losing 13 days of reading growth and 14 days of math at charter schools relative to kids receiving special education outside of charters. Raymond called that inequity one of the few sore spots revealed by the study, adding that charter schools should be “taken to task” for the collective failure.

“With the exception of very few charter schools that specialize in particular kinds of special education, the sector has basically thrown up their hands and said, ‘This isn’t our job,’” she said.

Even among charters, some types tend to yield better results than others. Specifically, those grouped within a charter management organization (CMO) — a network, either non- or for-profit, that operates multiple schools, such as the well-known KIPP or Success Academy organizations — provide 27 extra days of instruction in reading, and 23 extra days in math, than traditional schools. Stand-alone charters, which encompass roughly two-thirds of all charter schools, generate 10 extra days of reading growth and negative-three days of growth in math.

Douglas Harris, an economics professor at Tulane University the impact of charter schools on surrounding public school districts, said that the results of the CREDO report largely dovetailed with those of in New Orleans and elsewhere. He also said that the especially impressive findings from CMO-affiliated schools were somewhat predictable given that many cities and states only consider top-performing charter schools as candidates for replication.

“Some of this is kind of mechanical — not in a bad way, it’s just how the sector operates. If you’re a stand-alone, and you do well, you can open another school,” Harris said. “Then you become a CMO, and they’re better because they were selected to build on their own success. That’s a positive aspect of the charter model.”

Even more distinctive was the dividing line between what might be deemed “traditional” charters and those offering instruction virtually, which had already earned an ugly reputation for low academic quality even before the pandemic began. The popularity of the virtual charter sector has grown substantially since the emergence of COVID — by the Network for Public Education found that fully or mostly online programs enrolled 13 percent of all charter students during the 2020–21 school year — even as they delivered a staggering 124 fewer days of math growth than traditional public schools, along with 58 fewer days of growth in English.

If virtual initiatives were excluded from the national sample, the average charter school advantage would jump from 16 extra days of reading instruction to 21, and from 6 extra days of math instruction to 14. 

Martin West, the academic dean at the Harvard Graduate School of Education, called the report “easily the most comprehensive analysis of charter school performance to date” and echoed concerns about the performance of virtual charter schools.

“The results continue to raise questions about the regulatory environment for virtual charter schools, whose results drag down the overall performance of the broader sector,” West said. “These schools may provide an essential option for students for whom in-person learning truly isn’t possible, but state policymakers should look carefully at who is attending these schools and how well they are being served.”

An additional state-by-state analysis showed that individual jurisdictions have built particularly effective charter school sectors. Across New York State, charter students receive the equivalent of 75 extra days of growth in reading, and 73 extra days in math, compared with demographically similar students at district schools. Massachusetts (41 extra days in both subjects), Maryland (37 extra days in both subjects), Tennessee (34 extra days of reading and 39 in math), and Rhode Island (90 extra days of reading and 88 in math) offered similarly impressive statewide results. Charter school students only experienced significantly weaker reading growth in one state, Oregon.

An additional lesson came with respect to new charter entrants versus existing options. New schools opened by existing CMOs tended to outpace their district competitors, but also to be out-performed themselves by older schools within their own CMO.

“The new schools that have come in since the second study are strong, but they’re not as strong,” Raymond observed. “So it’s not that new schools are coming in and kicking butt and dragging the sector along with them. It’s that, over this period, individual schools around the country are making incremental changes that lead to this trajectory of upward performance.”

Now, as that restructuring has come under increasing scrutiny from local opponents, researchers say that it led to some of the most significant learning gains ever measured. A by scholars at the University of Colorado Denver finds that over a little more than a decade, the city’s schools transformed from one of the worst districts in Colorado to one that outperforms more than half the districts in the state. Four-year high school graduation rates leapt from 43 percent to 71 percent during the same period, and the progress was shared by a diverse array of student demographics.

The results offer powerful evidence in favor of the so-called “,” an educational strategy that began to take hold in major urban school systems in the mid-2000s. Deliberately conceived as an alternative to the traditional methods of American school governance, the approach emphasizes greater autonomy for educators while focusing district authorities on centralized functions like enrollment and transportation.

Denver was among the cities that fully embraced the model, experts have argued, making its successes particularly notable in education policy circles. Douglas Harris, an economist at Tulane University who has spent years studying reform efforts in New Orleans and elsewhere, said the new study had demonstrated proof of significant growth in Denver schools.

“The effects are clearly large,” Harris said. “Just as a loose approximation, if you leapfrog that many districts, clearly you’ve seen a lot of improvement.”

But that improvement was accompanied by fierce opposition among many Denver families, teachers, and public school advocates, many of whom spent years protesting the course pursued by district leaders. While the closure of dozens of low-performing schools engendered the greatest controversy, the detractors have also claimed that top-down direction from the superintendent and school board generally eroded the community’s faith in the system. Those complaints eventually cascaded into a successful campaign to “flip the board,” replacing reform-friendly members with a new majority that has viewed the portfolio strategy much more skeptically.

Parker Baxter — the study’s lead author and an energetic advocate of the Denver reforms — said that the central finding in his work was “simple and profound”: that learning undeniably increased for the average Denver student over the 11 years he studied, and that no group was harmed.

“The debate is framed in terms of whether these reforms helped or hurt the district overall, and this [study] provides the opportunity to evaluate that,” said Baxter, who worked at Denver Public Schools from 2008 to 2011. “The evidence we have is that students benefited from these reforms, even if they were not personally impacted by them.”

‘Remarkable’ range of positive results

To get at the impact of the portfolio shift, Baxter’s study dives into test score data from 2008 to 2019 — a period that encompasses most of Denver’s reform era.

That phase began a little earlier, of Superintendent Michael Bennet. Now a U.S. senator, Bennet spent three and a half years attempting to change a school system that ranked among the worst in the state. Under Bennet and his successor, Tom Boasberg, the portfolio model took shape.

Over 60 new schools were created in the decade that followed, while nearly 40 closed their doors permanently. Parents were presented with a bevy of novel school options, including a quickly expanding sector of charter and “innovation” schools. Those new offerings were integrated into a unified, district-wide application system that allowed students to freely select among different choices. 

Boasberg, who left the district in 2018 after nearly a decade to take on leadership of the Singapore American School, said that each of those alterations was an ingredient in the success of the reforms, but that the indispensable factor was far simpler: a focus on attracting and retaining better instructional talent to schools, whatever their particular type.

Ideological heat around school choice and accountability tends to obscure the single-minded focus on quality during the implementation of the reforms, Boasberg added.

“There’s a lot of political ideology around governance models,” he argued. “We really didn’t care about that. We were about: Are you a good school, and do you serve all kids? The governance model was not important to us.”

To what degree Denver’s improvements were attributable to the portfolio reforms can be debated, but their scale is impressive. Before the 2008–09 school year, Denver was one of the 10 lowest-performing school systems in Colorado on both math and reading tests, performing below the 5th percentile of districts statewide. In 2018–19, it had risen to the 60th percentile in reading and the 63rd percentile in math. In comparison with a group of similarly low-performing districts in the state, Baxter and his coauthors found, the reforms triggered growth equivalent to between 1 and 1.5 extra school years over the period studied.

That general progress spilled over into secondary areas, such as district enrollment, which increased by nearly 20,000 students between 2004 and 2019. While white children saw the largest gains overall, results were also positive for African American students in literacy. Hispanic and low-income students saw positive results in math and English, though they were not large enough to be considered statistically significant.

Baxter called the range of positive results, across both subject areas and racial categories, “remarkable.”

“The fact that we see significant positive results for students with disabilities, or for African American students in math — we would not necessarily expect a reform, even one that had such a positive impact systemwide, to also have these positive impacts for the most vulnerable subgroups.”

Denver Public Schools declined to comment for this story.

Model ‘hasn’t gotten very far’

Whatever the good news from the last decade of school governance, however, the next decade is much in question.

After the successful 2019 effort to flip the school board — replacing members who had largely backed Bennet and Boasberg’s approach with a new group that enjoyed more support from the local teachers’ union — a pronounced change in direction has taken place in the district. Superintendent Susana Cordova, a veteran of the reform regime who stepped in , soon left town herself after a brief tenure marked by poor performance reviews. The board’s new majority also voted to that had drawn criticism from educators. 

Nevertheless, three years into what might be deemed the “post-reform” period, many of the hallmarks of the portfolio model remain in place. The pace of school closures has slowed almost to a halt, but schools of choice still enroll a substantial portion of Denver students, and charter and innovation schools maintain wide autonomy in terms of hiring, curriculum, and scheduling. Boasberg said that the interlocking reforms embedded during his time in office would be difficult to do away with — if only because they remain broadly popular.

“The pieces do fit together, and that’s a really important part of it,” Boasberg said. “Why would you want to change the funding system to give less money to poor kids? Why would you want to have charters serve fewer English language learners and kids with special needs? Why would you want to take choices away from families?”

The future for the portfolio approach is perhaps murkier. After reaching a high point in the middle of the 2010s, school reform in major districts has stalled due to both political pressure and internal exhaustion. The model’s exemplars — New Orleans, which largely swept away the pre-reform landscape following Hurricane Katrina, is perhaps the prototypical example — have achieved substantial gains. But a large group of cities that attempted the portfolio pivot, from New York to Chicago to Washington, D.C., never completed the transformation.

Harris observed that, after years of hype and advocacy, the portfolio vision “hasn’t gotten very far.” That said, he added, its central ideas of expanded choice and unified district functions have left their mark in systems enrolling millions of students.

“Making structural changes in the education system is a very slow-moving enterprise, and the fact that we do have this idea — call if portfolio, call it what you will — that has infused a large number of urban districts, even in an impure form, is significant.”