Research suggests that early classroom experiences influence the socialization of aggression. Tracking changes in the aggressive behavior of 4,179 children from kindergarten to second-grade (ages 5–8), this study examined the impact of 2 important features of the classroom context—aggregate peer aggression and climates characterized by supportive teacher–student interactions. The aggregate aggression scores of children assigned to first-grade classrooms predicted the level of classroom aggression (assessed by teacher ratings) and quality of classroom climate (assessed by observers) that emerged by the end of Grade 1. Hierarchical linear model analyses revealed that first-grade classroom aggression and quality of classroom climate made independent contributions to changes in student aggression, as students moved from kindergarten to second grade. Implications for policy and practice are discussed.