Given that previous findings on the social distribution of the effects of small classes have been mixed and inconclusive, in the present study I attempted to shed light on the mechanism through which small classes affect the achievement of low- and high-achieving students. I used data from a 4-year, large-scale, randomized experiment (project STAR) to examine the effects of small classes on the achievement gap. The sample consisted of nearly 11,000 elementary school students who participated in the experiment from kindergarten to grade 3. Meta-analysis and quantile regression methods were employed to examine the effects of small classes on the achievement gap in mathematics and reading SAT scores. The results consistently indicated that higher-achieving students benefited more from being in small classes in early grades than other students. The findings also indicated that, although all types of students benefited from being in small classes, reductions in class size did not reduce the achievement gap between low and high achievers. (Contains 6 tables.)