Violence against children from school staff is widespread in various settings, but few interventions address this. The authors tested whether the Good School Toolkit - a complex behavioural intervention designed by Ugandan not-for-profit organisation Raising Voices - could reduce physical violence from school staff to Ugandan primary school children. Methods: The authors randomly selected 42 primary schools (clusters) from 151 schools in Luwero District, Uganda, with more than 40 primary 5 students and no existing governance interventions. All schools agreed to be enrolled. All students in primary 5, 6, and 7 (approximate ages 11–14 years) and all staff members who spoke either English or Luganda and could provide informed consent were eligible for participation in cross-sectional baseline and endline surveys in June-July 2012 and 2014, respectively. The authors randomly assigned 21 schools to receive the Good School Toolkit and 21 to a waitlisted control group in September, 2012. The intervention was implemented from September, 2012, to April, 2014. Owing to the nature of the intervention, it was not possible to mask assignment. The primary outcome, assessed in 2014, was past week physical violence from school staff, measured by students' self-reports using the International Society for the Prevention of Child Abuse and Neglect Child Abuse Screening Tool-Child Institutional. Analyses were by intention to treat, and are adjusted for clustering within schools and for baseline school-level means of continuous outcomes. Findings: No schools left the study. At 18-month follow-up, 3820 (92·4%) of 4138 randomly sampled students participated in a cross-sectional survey. Prevalence of past week physical violence was lower in the intervention schools (595/1921, 31·0%) than in the control schools (924/1899, 48·7%; odds ratio 0·40, 95% CI 0·26-0·64, p<0·0001). No adverse events related to the intervention were detected, but 434 children were referred to child protective services because of what they disclosed in the follow-up survey. Interpretation: The Good School Toolkit is an effective intervention to reduce violence against children from school staff in Ugandan primary schools.