What are the effects, if any, of smaller classes on student achievement? It depends on who you ask. The American Educational Research Association (AERA) published a brief round-up of research that suggested that there was some evidence for the efficacy of class size reduction. Drawing on the Tennessee Student/Teacher Achievement Ratio (STAR) study and the Wisconsin Student Achievement Guarantee in Education (SAGE) study, AERA forcefully concludes that classes of 13-17 students implemented in kindergarten or first grade are effective, especially for low-income students. Encouragingly, the results appear to stick, with students who had small classes in the early grades retaining an advantage though eighth grade. AERA does include caveats, pointing out that an increase in the number of teachers could lead to fewer students in the classrooms of effective teachers and also acknowledges that class size reduction may not be the most optimum use of limited resources.

A new RAND study came to more ambiguous conclusions. The study tried to evaluate the effect of California's 1996 "Class Size Reduction" initiative. Using achievement data from the Stanford-9 test given to California students annually between grades 2-11, RAND researchers found that elementary school scores did go up at the same time as the implementation of class size limit of 20. Unfortunately, they report that they were unable to sufficiently control for previous achievement to assert the cumulative effects of class size reduction, so they could not attribute gains to the initiative.