This study investigated the effect of a school-based violence prevention programme implemented in Grade 1 classrooms in Jamaican primary schools. Fourteen primary schools were randomly assigned to receive training in classroom behaviour management (n = 7 schools, 27 teachers/classrooms) or to a control group (n = 7 schools, 28 teachers/classrooms). Four children from each class were randomly selected to participate in the evaluation (n = 220 children). Teachers were trained through a combination of workshop and in-class support sessions, and received a mean of 11.5 h of training (range = 3–20) over 8 months. The primary outcomes were observations of (1) teachers’ use of violence against children and (2) class-wide child aggression. Teachers in intervention schools used significantly less violence against children (effect size (ES) = −0.73); benefits to class-wide child aggression were not significant (ES = −0.20). Intervention teachers also provided a more emotionally supportive classroom environment (ES = 1.22). No benefits were found to class-wide prosocial behaviour, teacher wellbeing, or child mental health. The intervention benefited children’s early learning skills, especially oral language and self-regulation skills (ES = 0.25), although no benefits were found to achievement in maths calculation, reading and spelling. A relatively brief teacher-training programme reduced violence against children by teachers and increased the quality of the classroom environment.