Background There is no consensus concerning the most effective type of facilitator to promote healthy sexual behaviours in interventions targeting adolescents. Objectives To evaluate the facilitator's effect on the implementation of the COMPAS programme (Spanish acronym for Skills for Adolescents with Healthy Sexuality), a school-based HIV prevention protocol. Methods Participants were 832 Spanish scholars aged 14 to 18. Fifteen schools were randomly assigned to one of the three following schemes: COMPAS delivered by experts only; the same programme administered by experts and peers; or a control group, not exposed to any intervention. Results The experts achieved an improvement in HIV knowledge and attitudes towards HIV and condom use; however, experts associated to peers only succeeded in increasing HIV knowledge. The effect size of the changes indicated a greater positive change in the programme when applied by experts than by experts and peers. Conclusions The participation of peers as co-facilitators did not increase the efficacy of a programme delivered by experts to Spanish adolescents. Education delivered by experts was the most effective modality for reducing sexual risk. COMPAS is the only Spanish programme targeting the promotion of safer sex behaviours in adolescents whose efficacy has been evaluated with different health agents in Spain.