A report recently published by the National Education Policy Center, titled "Class Size Matters" found that:

Class size is an important determinant of student outcomes, and one that can be directly determined by policy. All else being equal, increasing class sizes will harm student outcomes.
The evidence suggests that increasing class size will harm not only children’s test scores in the short run, but also their long-run human capital formation. Money saved today by increasing class sizes will result in more substantial social and educational costs in the future.
The payoff from class-size reduction is greater for low-income and minority children, while any increases in class size will likely be most harmful to these populations.
Policymakers should carefully weigh the efficacy of class-size policy against other potential uses of funds. While lower class size has a demonstrable cost, it may prove the more cost-effective policy overall.

Read the entire report below