Recent concerns about extremism, and young people’s vulnerability to exposure of radicalisation and such negative influences, have increased interest in young people’s participation in civic activities. There is some evidence that such activities at school can have a positive influence on young people’s attitudes and behaviour, but such evidence has been largely based on correlational, small-scale and somewhat biased studies. This paper presents the results of the first large independent randomised controlled trial in the UK to test the impact of participation in uniformed group activities in school on young people’s social and reported behavioural outcomes. The one-year trial involved 7781 thirteen to fourteen year olds across 71 secondary schools in England. Outcomes were measured before and after the intervention using a bespoke questionnaire survey. Attrition was negligible. The results showed positive ‘effects’ on a range of wider outcomes including self-confidence, teamwork, resilience, career aspirations, empathy and self-reported charitable activities. These effects are somewhat muted since not all pupils in the treatment schools actually took part in the intervention. Process evaluation suggests that the intervention was well-received, but strong leadership support is crucial for successful implementation. The findings provide evidence of the promise of the benefits of such uniformed group involvement for young people. If these activities are deemed worthwhile in their own right, because of the costs, then there is enough evidence here to pursue such a course.