Prevention programs, such as mindfulness-based interventions (MBIs), are often implemented in schools to prevent psychological disorders from emerging in children and to support their mental health. This study used a randomized cluster design to evaluate the impact of a MBI, called Mission Méditation, on the well-being and the mental health of elementary school children's. 13 classrooms of an elementary school were randomly allocated to the experimental condition (7 classrooms, n = 127 students) or the waitlist control condition (6 classrooms, n = 104 students). Participants in the experimental condition received a 10-week MBI. Regression analyses revealed significant differences between conditions for inattention. Participants in the MBI condition reported no change in pre- to post-intervention, whereas participants in the control condition reported pre- to post-intervention increases. Results also showed significant differences in perceived competence. Participants in the MBI condition reported a non-significant decrease in perceive competence, whereas participants in the control condition reported significantly higher perceive competence scores from pre- to post-intervention. Results do not indicate that the MBI had a significant impact on participant's well-being and mental health. This suggests that MBIs may not have an added value when compared to other preventive interventions geared towards well-being and mental health promotion in school settings.