Teachers promote student learning and well-being in school by establishing a supportive classroom structure. The term "classroom structure" refers to how teachers design tasks, maintain authority, and evaluate student achievement. Although empirical studies have shown the relation of classroom structure to student motivation, achievement, and well-being, no prior investigations have examined the influence of classroom structure on aggression among peers. The present study examined whether a supportive classroom structure has an impact on verbal and physical aggression. At two points in time, data were collected from 1680 students in Grades 5 to 7 using self-report questionnaires. The results of structural equation modeling revealed that a supportive classroom structure at Time 1 was associated with less perpetrated verbal aggression at Time 2, 9 months later. This finding has practical relevance for teacher training as well as for aggression prevention and intervention among children. (Contains 3 tables and 4 figures.)