Background Integrated programmes addressing varying forms of violence and abuse are increasingly delivered to children under 12 but uncertainty remains about what should be delivered to whom, when and in what dose. Objective To examine the impact of Speak Out Stay Safe (SOSS) - an integrated prevention programme for children under 12 - and whether impact varied by age, gender and context. Participants and setting A representative UK sample of primary schools in receipt of SOSS was matched with comparison schools not receiving SOSS. At 6 months follow-up, 1553 children from 36 schools completed the survey. Methods The matched control study incorporated economic and process evaluations. Survey measures included: children's knowledge and understanding of different forms of violence and abuse, readiness to seek help, knowledge of sexual abuse, perceptions of school culture and health and wellbeing. Perceptions of children, teachers, and facilitators were captured. Results At 6 months, children aged 9–10 who received SOSS retained their improved knowledge of neglect and their ability to identify a trusted adult who they would tell about violence or abuse. Children aged 6–7 receiving a shorter version of the programme were less likely to benefit and boys made fewer gains than girls. SOSS improved the knowledge of children with low knowledge of abuse. School culture was closely associated with programme impact. Conclusion School-based prevention programmes deliver benefits at low cost but should acknowledge and engage with the specific school context to achieve school readiness and embed programme messages.