Iain Murray remarks that there is "no evidence that class sizes have any real effect on the quality of education". I've heard that before, and my experience in Taiwan years ago was that despite the huge class size, students did well. Here's evidence beyond the anecdotal:

The Class Size Reduction (CSR) evaluation was conducted by a research consortium comprising the American Institutes for Research (AIR), RAND, WestEd, Policy Analysis for California Education (PACE), and EdSource.

Among other things, it says:

Our analyses of the relationship of CSR to student achievement was inconclusive. Student achievement has been increasing since the first administration of the SAT-9 in 1997, but we could find only limited evidence linking these gains to CSR.

But,

CSR is an enormously popular program in California among elementary parents and teachers. It is also clear that local educators and parents may value reduced class sizes for many reasons other than improved achievement as measured by statewide test scores. Therefore, maintaining small K�3 classes in California is likely to remain a priority.

Well, as they say, politics is the art of the possible. But it's infuriating that people want something that doesn't do much good, but costs a lot of money.

Erik Hanushek argues that "the evidence about improvements in student achievement that can be attributed to smaller classes turns out to be meager and unconvincing....While policies to reduce class size may enjoy popular political appeal, such policies are very expensive and, according to the evidence, quite ineffective."