In the Sunday Times, Sara Mosle suggests that the way to square the circle on the class size debate--while working within current budget limitations--is to guarantee smaller classes for poor children only, while setting up pilot projects to test the thesis of Bill Gates, Arne Duncan, Mike Bloomberg, and Mitt Romney: That excellent teachers might be willing to take on 3-5 more students per class for a moderate pay raise, say $5,000 or $10,000. The reformers argue that improved student achievement would be the result.

The political problem with Mosle's idea is that many of the loudest advocates for small classes are middle-class and affluent parents. This is backed by polling; by a growing national advocacy movement, driven by middle-class parents and educators, that considers small classes a sacred right for all kids; and by evidence that when American parents choose private schools, one of the main perks they are purchasing is lower teacher-to-student ratios. In fact, the United States has one of the largest private school-public school class size gaps in the developed world -- 19.4 vs. 23.6 children -- compared to an average of just one additional student in the public school classrooms of other developed nations.

via the OECD

In other words, denying middle-class kids smaller classes turns out to be a great way to alienate middle-class parents from education reform, and might even drive rich families out of the public school system entirely. This is problematic for the political sustainability of education reform.

There's a policy problem, too: Pursuing smaller class sizes only for disadvantaged children assumes that poor and middle-class kids are sitting in different classrooms or attending different schools. This is true most of the time, yet because of demographic shifts in both urban and suburban areas, we are seeing more and more diverse classrooms -- a good thing. Giving districts extra money for class size reductions targeted at poor kids could induce schools to track disadvantaged children into separate classes, something that actually happened during the Great Society era when federal funds were first tied to efforts to provide poor children with extra services. 

Click here to read about research on the relationship between class sizes and student achievement.