Numerous classroom-based interventions have aimed to improve sexual health outcomes for young people, yet few have shown strong, lasting effects. Ecological approaches that address multiple levels of a young person's environment offer largely untapped potential to positively change sexual behaviour. This paper presents results of a cluster-randomised trial of a multicomponent sexuality education intervention that integrated a classroom curriculum, parent education workshops and materials, peer advocate programme, and sexual health services at 10 urban high schools. An implementation evaluation, employing quantitative and qualitative instruments, was conducted to examine whether the individual components of the intervention were delivered as planned and how they were received by the target populations. Multilevel modelling was used to evaluate the effect of the multicomponent intervention on outcomes at one-year follow-up. Results showed that the intervention components were successfully implemented with and well received by target populations. Students receiving the multicomponent intervention reported greater increases in the use of sexual health services (odds ratio [OR] = 1.73, 95% CI = 1.09-2.75) and the likelihood of carrying a condom (OR = 2.71, 95% CI = 1.44-5.09) relative to those receiving a control condition. No effects were found for other behaviours, possibly due to low prevalence of sexual activity in the sample and the small number of schools randomised.