ABSTRACT Background Less than 20% of adolescents globally meet recommenced levels of physical activity, which comes with social disadvantage and rising disease risk at increasingly early ages. The determinants of physical activity in adolescents are multilevel and poorly understood, but the school’s social environment likely plays an important role. We conducted a cluster-randomised controlled trial to assess the effectiveness of a school-based programme (GoActive) to increase moderate-to-vigorous physical activity (MVPA) among adolescents.

Methods/Findings Non-fee-paying, co-educational schools including Year 9 students in the UK counties of Cambridgeshire and Essex were eligible for inclusion. Within participating schools (N=16), all Year 9 students were eligible and invited to participate. Participants were 2862 13-14-year-olds (84% of eligible students). After baseline assessment, schools were computer-randomised, stratified by school-level pupil premium (below/above county-specific median) and county (control: 8 schools, 1319 participants, mean (SD) participants per school: n=165 (62); intervention: 8 schools, 1543 participants, n=193 (43)). Measurement staff were blinded to allocation. The iteratively-developed, feasibility-tested 12-week intervention, aligned with Self-Determination Theory, trained older adolescent mentors and in-class-peer-leaders to encourage classes to conduct two new weekly activities. Students and classes gained points and rewards for engaging in any activity in- and out-of-school. The primary outcome was average daily minutes of accelerometer-assessed MVPA at 10-month follow-up; a mixed-methods process evaluation evaluated implementation. Of 2862 recruited participants (52.1% male), 2167 (76%) attended 10-month follow-up measurements; we analysed the primary outcome for 1874 participants (65.5%). At 10-months, there was a mean (SD) decrease in MVPA of 8.3 (19.3) minutes in the control group and 10.4 (22.7) minutes in the intervention group (baseline-adjusted difference [95% confidence interval] -1.91 minutes [-5.53 to 1.70], p=0.316). The programme cost £13 per student compared with control; it was not cost-effective. 62.9% of students and 87.3% of mentors reported that GoActive was fun. Deliverers commented that their roles in programme delivery were unclear. Implementation fidelity was low. The main methodological limitation of this study was the relatively affluent and ethnically homogeneous sample.

Conclusions In this study, we observed that a rigorously developed school-based intervention was no more effective than standard school practice at preventing declines in adolescent physical activity. Interdisciplinary research is required to understand educational setting-specific implementation challenges. School-leaders and authorities should be realistic about expectations of effect of school-based physical activity promotion strategies implemented at scale.