We know that the best teachers get incredibly more out of their students than do the worst teachers. So why not assign more students to the best teachers? While the best teachers probably would do a little less well with bigger classes (although would be partially offset by the weak teachers doing a bit better with smaller classes), the effect of class size is small compared to the effect of teacher quality.
Mike Hansen puts together some very nice numbers on what could be gained by sorting more students toward good teachers. What he finds is that the effects are pretty encouraging overall. I would characterize the gains as nice, although not overwhelming. Hansen also shows that this kind of policy change would do less for economically disadvantaged students.
Here’s the experiment: In each school assign students to the best teacher until the point at which the larger class size effect begins to outweigh the teacher being really good. Then start assigning students to the second best teacher, and so on. Here’s how Hansen describes the outcome.
Intensively reallocating eighth-grade students—so that the most effective teachers have up to twelve more pupils than the average classroom—may produce gains equivalent to adding roughly two-and-a-half extra weeks of school. Even adding a handful of students to the most effective eighth-grade teachers (up to six more than the school’s average) produces gains in math and science akin to extending the school year by nearly two weeks or, equivalently, to removing the lowest 5 percent of teachers from the classroom. The potential impacts on learning are more modest in fifth grade…
Hansen also writes “this policy alone shows little promise in reducing achievement gaps.” The problem is that economically disadvantaged students are largely clustered in schools where there just aren’t enough great teachers to make much of a difference, no matter how you juggle student assignments.
One more important point, Hansen suggests that differential class sizes provides an opening for paying more to more effective teachers in a setting that might be relatively palatable.
…one of this strategy’s unique features is that it’s a way of paying high-performing teachers more—under the cover of giving them more students….The policy outlined here simply suggests being selective about which teachers receive the extra students (and extra pay).