Objectives: This study sought to evaluate the efficacy of a brief version of the Happy Classrooms Programme in psychological well-being, school aggression, and positive classroom climate. Likewise, this study also aimed to identify which intervention effects could be attributed to the development of mindfulness. Finally, the last target of this study was to evaluate the implementation fidelity and the acceptability of the programme.Methods: Multiple linear regression and longitudinal mediation analyses were performed with a sample of 524 students (49.8% boys and 50.2% girls) with a mean age of 13.6 years.Results: Results provided evidence of the efficacy of the intervention in the majority of variables. For