Bullying is a widespread form of aggression that emerges early in childhood and is common in school settings. Using Italian data from the National Institute for the Evaluation of Education and Training (INVALSI) on primary school students, we document gender differences in self-reported bullying, both as victims and perpetrators, across multiple dimensions. Bullying is more prevalent among boys on both fronts. Exploiting the quasi-random allocation of students to classes within schools, we show that a higher share of boys increases reported victimization among girls, particularly in forms such as mockery and verbal insults. These effects are associated with lower well-being among girls. The findings point to a spillover of violence from boys to girls as the share of male peers increases, highlighting the role of classroom gender composition in shaping early peer interactions and the need for caution when managing gender imbalances in elementary education.