This study examines seasonal change in child aggressive behavior over 2 calendar years and explores the role of classroom composition on developmental trajectories. Four waves of data were collected in the fall and spring of 2 academic years from a sample of children attending New York City public elementary schools. Using the school calendar year as a reference point, we estimate average rates of change in aggression during the periods in which schools are in session as well as for the summer break. Employing centering strategies, we also explore the effect of different individual and classroom-level distributional positions on the estimated trajectories. Findings support the existence of an important "summer drop" in aggression that contrasts with positive growth during the school year. We also find main and moderated differences in these trajectories associated with the position students and classrooms occupy within the school and sample distribution of aggressive behavior.