The present study examined how the supportive climate of a school (group support and academic support) and student mental health (depression and anxiety) are associated with school-based aggression and victimization. Drawing on the social identity perspective, school identification (i.e., a person’s social identity as a school member) was investigated as a mechanism to explain how the supportive climate of a school may come to affect peer aggression and victimization. Using data from Australian students (Grades 7–10, N = 1,809) negative binomial structural equation modeling showed that (a) school climate factors and school identification negatively predicted peer aggression, (b) depression and anxiety were also strong predictors of peer aggression, and (c) school identification partially mediated the association of group support (but not academic support) with peer aggression. A different pattern was observed for peer victimization. The findings have a range of implications for addressing school-based aggression highlighting to school leadership and education policymakers that building and strengthening a supportive climate and school identification will be fruitful. (PsycINFO Database Record (c) 2018 APA, all rights reserved)