Peer violence in schools is a worrying phenomenon, especially the potential long-term impact it may have on a child’s future. Here we evaluate the effects of a programme of social mediation at school implemented to reduce peer violence. Through a randomised experimental design, we show that the effects of the programme are very important when it is implemented by mediators who are over 25 years old: in middle school, the rate of harassment experienced by pupils is reduced by 11%. Specifically, the programme is extremely beneficial for pupils most exposed to violence: the probability of feeling harassed decreased by 46% in boys in 6th grade and by 26% in girls in 7th grade. Interesting effects on the psychological and social well-being of students, as well as on student absenteeism and teachers are also visible.