Could raising charter cap help district school students?

ON NOVEMBER 8, Massachusetts voters will decide whether to lift the cap on charter schools operating in the state. Measure 2 is surely the most hotly contested issue on the ballot, attracting large sums of cash and heated rhetoric on both sides. CommonWealth has already published several essays for and against raising the cap—but there is additional research evidence that is relevant to the debate and has not yet been given public attention.

Prominent scholars have shown that in the urban areas that have hit the cap, charter schools are doing a great job improving their students’ achievement. These studies, conducted by teams of researchers from Harvard, MIT, and other universities, are rock solid. They use methods that approach the scientific rigor of randomized clinical trials required by the FDA to test new drugs. Even so, voters need more information: The fact that the state’s urban charter schools are good for their own students doesn’t tell us whether raising the cap would be good for all of the students in those communities.

Indeed, the indirect effect of charter schools has been—appropriately—a central part of the debate. When students leave district-run schools for charter schools, public funding goes with them. Opponents of raising the cap—including Boston mayor Martin Walsh—reasonably worry about the harm that could result to district schools that lose funding. Supporters counter by pointing out that district schools lose funding only in proportion to the number of students who depart, so that their per-pupil funding is not reduced.

But the debate over indirect effects has been almost entirely about the flow of funds. The two sides have different implicit assumptions about how the transfer of funds ultimately affects students who remain in district schools.

Opponents of raising the cap worry that students in district schools will be harmed by the loss of funds and perhaps by the loss of motivated students and engaged parents. Charter-school supporters, in contrast, focus on the fact that districts keep all of the funding for the students they continue to serve, and expect that districts will adjust appropriately to smaller enrollments. Moreover, proponents of charter schools sometimes argue that charter schools create healthy competition for district schools, potentially benefiting the students in district schools if the districts respond constructively to the competition.

Both of these competing hypotheses about the indirect effects of charter schools are plausible. But which one is true? Fortunately, we don’t need to rely entirely on conjecture to address the question. There is a body of evidence from existing research on the effects of charter schools on students in nearby district schools.

My colleague Kevin Booker and I conducted a comprehensive review of this evidence for the Handbook of Research in Education Finance and Policy. Eleven studies of the indirect effects of charter schools have been conducted using data that follows individual students in district schools over time, assessing how their achievement is affected when charter schools open or grow nearby. To be sure, measuring indirect effects is difficult, posing serious methodological challenges. These studies, therefore, are not as definitive as the Harvard/MIT studies of the direct effects on charter students. And none of the 11 studies specifically focused on Massachusetts. Nonetheless, the studies, which collectively include 11 different cities and states along with one nationwide sample, provide the best evidence available on charters’ effects on students in district-run schools.

The bottom line is this: Across the 11 studies, there is some evidence supporting the “healthy competition” hypothesis and almost none indicating that a resource drain harms students who stay in district-run schools. Specifically, six studies found some positive effects of charter-school competition on students in nearby district schools; four studies found no clear effects, positive or negative; and only one study in one (unnamed) school district found evidence of negative effects.

In support of the “healthy competition” hypothesis, there are clear examples of communities where student outcomes in district schools have improved at the same time that the charter sector has grown. In Washington DC, charter-school enrollment has surpassed 40 percent of all public-school students, while achievement levels in district schools have increased rapidly and the district’s graduation rate has broken historical records. New York has likewise seen citywide improvements in student outcomes while hosting a thriving charter sector (and borrowing ideas from that sector for its own schools). Here in Massachusetts, as charter school enrollments have grown in Boston, the district’s results on the National Assessment of Educational Progress have improved, and graduation rates in the Boston Public Schools have reached all-time highs.

Not every city has seen improvements in district schools as charter schools have grown. But the research suggests that the results in Boston, New York, and Washington, DC, are representative of the broader pattern: Most studies have found neutral or positive rather than negative indirect effects of charter schools on students in district schools. Upward trends in district schools in Boston and elsewhere demonstrate the encouraging possibility that raising the charter school cap does not have to mean helping some kids at the expense of others. Instead, the growth of charter schools may ultimately serve the interests of charter students and district students alike.

Brian Gill is a senior fellow at Mathematica Policy Research in Cambridge.

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