Reducing class sizes has long been championed as a way to improve outcomes for students.

Supporters – including many parents and teachers – argue that smaller classes allow students to get more individual attention and result in better grades, better test scores and fewer disciplinary issues. But critics of class-size reduction efforts argue that such reforms simply take money away from other priorities without making a meaningful difference in children’s learning.

“No one’s going to argue against the substantive argument for reducing class sizes, right?” says Douglas Ready, a professor of education and public policy at Columbia University in New York. “It sounds great. Teachers want it. Parents want it. Everybody wants it. The issue is paying for it and finding teachers to do it.”

Benefits and Challenges of Smaller Classes

Unlike other areas of education research, Ready says, there have been large-scale, randomized control trials – generally considered the gold standard in research design – examining the effects of class-size reduction. These studies found that smaller classes correlate with better test scores.

In 1985, Tennessee launched an experiment, the Student/Teacher Achievement Ratio (STAR) project, in which it assigned 7,000 kindergarten students in 79 schools to classes of varying sizes.

After four years, the students who had been placed in small classes were between two and five months ahead of their peers in larger classes, according to a report on the study in the academic journal Teachers College Record. Even once the experiment ended and students returned to full-size classrooms, students who had been in the smaller groups continued to benefit. By eighth grade, they were almost a full school year ahead of their peers.

Then in 1996, Wisconsin conducted a similar experiment targeting schools serving low-income students and comparing adolescents in classrooms of 12 to 15 with students in classrooms of 21 to 25 students. Here again, students in smaller classes achieved higher test scores, according to a federal examination of the study.

“The research is crystal clear that smaller classes lead to better student outcomes in every single way that can be measured,” says Leonie Haimson, executive director of Class Size Matters, a nonprofit that advocates for smaller class sizes.

But results from more recent studies using different methods have been mixed.

For one, reducing class sizes means hiring more teachers, which has long been a challenge for districts, Ready says.

A study of class-size reduction in New York City public schools from 2009 to 2013 found that many of the gains in test scores were offset by declines due to the “new teacher effect.” The report concludes that reducing class sizes can “substantially improve student achievement,” but only if schools can do so without affecting teacher quality.

In 1996, California launched an initiative to reduce class sizes by providing $650 to districts for every student in a kindergarten through third grade classroom with 20 or fewer students. Districts quickly hired about 30,000 new teachers, and the program cost the state billions of dollars.

Many of those educators were inexperienced and uncertified, according to a report from the Public Policy Institute of California. And the problem was particularly acute for Black students at schools with predominantly low-income students, where nearly 25% of students had a teacher with two years of experience or less, and 30% had a teacher who was not fully credentialed. Among white students at schools with fewer poor students, only 12% of students had a teacher with two years of experience or less, and only 5% had a teacher who was not fully credentialed.

Still, an analysis of studies on the class-size reduction program show that it “had a positive and significant influence on student achievement,” according to a Princeton University researcher. “Black students seem to have benefited” from class-size reduction “more than any other racial or ethnic group.”