William J. Mathis has drawn together the research on class size to explain why it matters. 

It is costly to reduce class size, but it is very likely the most effective intervention to help students who are struggling.

It also happens to be the single reform that parents want most. When class size was put on the ballot in Florida, it was overwhelmingly approved. Despite numerous attempts by Jeb Bush and his allies to get rid of it, the caps on class size have remained intact.

Reformers disregard the research on class size because they don’t want to spend more money to do what works. They prefer changes in governance, like charter schools, vouchers, mayoral control, state takeovers–anything but reducing class size. They claim that reducing class size benefits unions because it requires more teachers. But the biggest benefit of reducing class size is to the children, who get the attention and time they need to learn.