The last time I looked into the empirical evidence on this in primary school, the finding was that class size makes a big difference if there are 13 or fewer students in a class because the teacher can use a style where he spends individual time with each student. But from 15 to 40 it flat lines because the teacher has to switch to lecture mode. So, unless you can reduce class size to 13, there's no benefit to doing it. And there really aren't any schools that are taking it that far. So they might as well ramp up to 40 students (above this discipline and control becomes a problem in K-12) and use the salary savings to buy textbooks or other materials, things that many school districts are now opting out of because their administrative, salary and other such costs are now so high.

Many countries that do better achievement wise than the US (assuming measured achievement results are valuable, which is debatable) have large classroom sizes in primary education. Japan's average is 40 students:

> The 1999 Pennsylvania State University study done by Suetling Pong concluded that the effect of class size on achievement is very small. It compared ten industrialized nations other than the U.S. and found that students in Australia, Flemish Belgium and France did significantly better in larger math classes. Class size had no effect on students in Canada, Germany, Iceland, South Korea and Singapore. And students in Japan, who consistently outscore U.S. students in math and science, frequently attend math classes of 40 or more students. source: http://www.edreform.com/Archive/?Debunking_the_Class_Size_My...