We reviewed the effects of mindfulness-based school interventions (MBSIs) on youth outcomes. We evaluated seventy-three studies with a total sample a total of 11,906 students across five continents, assessing the quality of each study through a robust coding system for evidence-based guidelines. Coders rated studies as 1++ (systematic review) to 4 (expert opinion) for level of evidence. Outcomes were assigned a corresponding evidence quality letter grade, from strongest (A) to weakest (D) evidence. Outcomes fell into 11 categories: wellbeing, self-compassion, social functioning, mental health, self-regulation and emotionality, mindful awareness, attentional focus, psychological and physiological stress, problem behaviors, academic performance, and acceptability. Strongest evidence showed increased resilience and reduced anxiety. There was comparable improvement in depression and wellbeing across youth relative to control groups. We urge researchers interested in MBSIs to study their effectiveness using more rigorous designs to minimize bias and promote higher quality evidence to guide school-based practice.