Research shows that teacher quality trumps class size. (See Linda Darling-Hammond and Michigan State's Education Policy Center.) The strongest evidence suggests that class size only makes a difference in the early grades. Smaller classes are political popular -- and often demanded by parents -- but what is most important is who is teaching the class, not how many students are in it.
Jan Mathews writes:
Let's pretend Fairfax County schools get a surprise $44 million from the federal stimulus package this summer. With that money, the school system could make each class, on average, two students smaller, or it could do what some high-achieving schools do: Keep class sizes large and focus instead on more energetic recruiting and training of teachers. Research indicates that a two-student reduction would make little difference. Why not see what better instruction could do?
Or, try this thought experiment: The principal says your child can be transferred to the school's best teacher, an imaginative and energetic motivator, but that will push the class's size up to 30. Would you decline the offer? I wouldn't.
Then, there is California's sobering experience (CSR Research Consortium Capstone Report, 2002) when it reduced class sizes across the board in the 1990s. Because smaller classes require more teachers, California's policy depleted the teacher pool so incredibly that many more students were being taught by unqualified, unlicensed instructors. Not the desired outcome, most certainly.
Here's some good analysis and implications for ESEA re-authorization hot off the presses, so to speak, from Teacher Beat.