Note: Abstract updated in March 2024: Globally, 150 million adolescents report being victims of or engaging in peer-to-peer violence in and around school. One strategy to reduce this risk is to occupy youth in afterschool programs (ASP). Yet, the question remains: how does peer group composition affect the effectiveness of an ASP? This paper addresses this question by randomly assigning youths to either a control, homogeneous, or heterogeneous peer group within an ASP implemented in El Salvador. Results indicate that, unlike homogeneous groups, heterogeneous peer groups do help students avoid violence. These results are relevant to public policy discussions on optimal group composition for violence reduction programs