This systematic review and meta-analysis examines the effects of mindfulness-based interventions (MBIs) in schools on psychological outcomes. The study identified 24 relevant studies, 13 of which were published, with 19 using a controlled design. A total of 1,348 students were instructed in mindfulness, while 876 served as controls, ranging from grades 1 to 12. Overall effect sizes were Hedge’s g = 0.40 between groups and g = 0.41 within groups (p < 0.0001). The most significant effects were observed in cognitive performance (g = 0.80), stress (g = 0.39), and resilience (g = 0.36). Emotional problems (g = 0.19) and third-person ratings (g = 0.25) showed smaller, non-significant effects. The review highlights the promise of MBIs in improving cognitive performance and resilience to stress in children and youths, but also notes the heterogeneity and underpowered nature of many studies, which require careful examination. The field is still in its early stages, and recommendations are provided for future research and intervention development.This systematic review and meta-analysis examines the effects of mindfulness-based interventions (MBIs) in schools on psychological outcomes. The study identified 24 relevant studies, 13 of which were published, with 19 using a controlled design. A total of 1,348 students were instructed in mindfulness, while 876 served as controls, ranging from grades 1 to 12. Overall effect sizes were Hedge’s g = 0.40 between groups and g = 0.41 within groups (p < 0.0001). The most significant effects were observed in cognitive performance (g = 0.80), stress (g = 0.39), and resilience (g = 0.36). Emotional problems (g = 0.19) and third-person ratings (g = 0.25) showed smaller, non-significant effects. The review highlights the promise of MBIs in improving cognitive performance and resilience to stress in children and youths, but also notes the heterogeneity and underpowered nature of many studies, which require careful examination. The field is still in its early stages, and recommendations are provided for future research and intervention development.