Concerns regarding the social-behavioral maladjustment of U.S. youth have spurred efforts among educators and policymakers to identify and remedy educational contexts that exacerbate children's anxiety, depression, aggression, and misconduct. However, investigations of the influence of collective classroom student characteristics on individuals' social-behavioral functioning are few. The present study examined concurrent and longitudinal relations between adversity factors facing the collective classroom student group and levels of children's internalizing and externalizing behaviors across the elementary school years, and whether the pattern of relations differed for girls and boys. First-, third-, and fifth-grade teachers reported on the extent to which adversity-related factors (e.g., home/family life, academic readiness, social readiness, English proficiency, tardiness/absenteeism, student mobility, health) presented a challenge in their classrooms (i.e., "classroom-level adversity" [CLA]). Mothers reported on their child's internalizing and externalizing behavior at each grade. Autoregressive, lagged panel models controlled for prior levels of internalizing and externalizing behavior, mothers' education, family income-to-needs, and class size. For all children at each grade, CLA was concurrently and positively associated with externalizing behavior. For first-grade girls, but not boys, CLA was also concurrently and positively associated with internalizing behavior. Indirect effects suggested CLA influenced later internalizing and externalizing behavior through its influence on maladjustment in a given year. Discussion highlights possible methods of intervention to reduce CLA or the negative consequences associated with being in a higher-adversity classroom.