This study assessed the impact of a school-based program aimed at preventing disordered eating. The program was based on the media-literacy approach and has interactive format. The program was assessed under strong methodological conditions. Seven schools with 263 Spanish adolescent girls in the area of Barcelona, were randomly assigned to either the complete prevention program condition, the partial program condition or the non-treatment control condition, and assessed at pre, post and 6-month follow-up. The program was effective in generating positive changes at follow-up. The effects sizes (ES_d= 0.29 to ES_d= 0.38) were greater, on average, than that obtained up to now in selective-universal programs, and similar or greater than that achieved by targeted prevention programs. The results indicate a greater and relevant effect size of the intervention in those participants who completed the inter-session interactive activities (ES_d= 0.29 to ES_d= 0.45) although the differences were not significant. These results suggest the importance of monitoring adherence to the activities in all programs defined as “interactive”. The implications and limitations of this study are discussed.