Various factors at the individual and school levels contribute to the impairment of academic achievement. We examined whether academic class composition, measured as average class achievement and class heterogeneity (variance in achievement within class), moderates the associations of inattentiveness and hyperactivity/impulsivity with academic achievement. Data from a two-wave longitudinal study of Russian first graders (N = 2,605, 51% girls) were used. Two-level regression analysis revealed inattentiveness to be negatively associated with math and reading achievement and hyperactivity/impulsivity to be positively associated with achievement but only under control for inattentiveness. The results reveal that the associations of inattentiveness and hyperactivity/impulsivity with math achievement did not vary in classes of different academic compositions. However, the negative effect of inattentiveness on reading decreased in classes with high average achievement. The results also demonstrated that, controlled for inattentiveness, hyperactivity had a stronger positive association with reading in heterogeneous, rather than homogeneous, classes.