Abstract The high global prevalence of school violence underlines the need for prevention. However, there are few scientifically evaluated intervention approaches that aim at preventing violence by teachers. We evaluated the feasibility and efficacy of the preventative intervention Interaction Competencies with Children for Teachers (ICC-T). In a cluster randomized controlled trial we assessed attitudes towards and use of violence by teachers (self-reported and reported by students) at eight schools in four regions in Tanzania. Two regions were randomly assigned as intervention regions. Data were assessed in the months before and three months after intervention. In total, 158 teachers (58% females; age: 32.08 years, SD = 5.65) and 486 students (54% females; age: 15.61 years, SD = 0.89) participated in this study. The feasibility was very good: Participants’ acceptance was high and they reported a good integration of the core elements in their working routine. The significantly stronger decrease in the use of emotional and physical violence reported both by teachers and students as well as the stronger decrease in positive attitudes of teachers towards physical and emotional violence in the intervention schools at follow-up provide initial evidence of the efficacy. However, further evidence for the sustainability of its effect is needed.Scholar Fund of the University of Konstanz and vivo Iinternational.