Efforts to racially integrate schools in big city districts often face a basic dilemma: There simply aren’t many white students in the system, as families have opted for private school or left for the suburbs.   

But a recent study suggests a concrete way that schools can attract and keep white families, while also boosting student achievement: lower class sizes. That approach drew in tens of thousands of students from California’s private schools into the public system, according to the research.

It’s an extremely expensive move, since it means hiring more teachers for more classrooms. And it’s far from clear to what extent lower class sizes can combat how racism and beliefs about local schools drive enrollment decisions, particularly in racially segregated areas.

Still, the California results suggests that lower class sizes can be one tool to help integrate schools.

“Class size is a measure of school quality that is easily observable [and] parents care about it a lot,” said Diane Whitmore Schanzenbach, an economist at Northwestern University who has studied class size reduction efforts. It’s “completely reasonable,” she said, to think that lower class sizes could help make integration efforts work.

The research, released earlier this year through the National Bureau of Economic Research, focuses on California’s massive effort to slash elementary school class sizes by several students in the late 1990s. (The program began to unravel in the wake of the Great Recession, when budget cuts caused many districts to rapidly raise class sizes.)

Previous research had focused on how the effort affected student achievement and teacher quality. This study asks an additional question: Did the reforms make public schools more attractive to families sending their kids to private schools?

Researchers Michael Gilraine, Hugh Macartney, and Robert McMillan find that’s exactly what seemed to happen. In grades that saw class sizes drop, private school enrollment fell compared to other grades without the class-size cuts.

The drop was fairly large, with private school enrollment falling from nearly 12 percent to 10 percent of students in certain grades across the state. At the same time, public schools with a nearby private school saw their enrollment of white students jump.

As students transitioned to middle school, which was not a focus of the class size reductions, many students appeared to return to private school.

Conventional wisdom in some circles has been that the California class size reduction was a disappointment because districts had to go on a hiring spree to fill new classrooms, ending up with less qualified and less experienced teachers.

The latest study shows there’s something to that: teacher qualifications fell, which cancelled out some, but not all, of the benefits of smaller classes. That’s in line with other research from California and New York City.

The paper also shows that test scores likely rose simply because schools drew in higher-performing students from private schools.

But there was another result of the changes: the new public school students seemed to boost other kids’ test scores. Although the exact explanation is unclear, this might be an effect of integration — an example of how bringing new students into the public system can improve education across the board.

Still, before policymakers use this study, which has not been formally peer-reviewed, to justify reducing class sizes, they should consider a few caveats.

For one thing, the paper focuses on a policy change in one state from about two decades ago. While it shows how the changes affected the demographics of the public school system as a whole, it doesn’t examine the makeup of individual schools. And the study doesn’t have ideal or extensive data for examining test scores, so those academic findings should be interpreted cautiously.

It’s also just one analysis, in an area where there’s not much research. Another recent study on New York City found that increasing public school funding drew in more students who previously attended private school, though the students were primarily black and Hispanic, not white.

More generally, research has often found that smaller class sizes have positive short- and long-term benefits, but results have varied from place to place.

The policy could mean schools districts are spending a lot, perhaps even more than they expect. If lower class sizes cause students to leave private schools, it will also come with an ever bigger price tag, because public dollars would need to go to educating students who had been at private school on their families’ dime.

In other words, the study shows smaller class sizes may be both more beneficial and more costly than previously thought.

“Without any doubt, [class-size reduction] is immensely costly,” the researchers write. “That said, parents and teachers routinely and actively lobby for smaller classes, pressuring politicians to implement class size reduction initiatives.”