School-based physical activity and adolescent mental health: candidate psychological mechanisms, contextual moderators, and unresolved controversies.

Adolescence is a peak period for the onset of mental disorders, yet the majority of young people fail to meet recommended physical activity levels. This convergence of rising psychological distress and declining physical activity has positioned schools, settings that reach nearly all adolescents, as a central platform for population-level mental health promotion through movement. This mini review synthesizes recent evidence on the effects of school-based physical activity interventions on adolescent mental health, focusing on the psychological mechanisms that carry these effects, the contextual factors that modify them, and the controversies that remain unresolved. Evidence is kept in three tiers throughout: trials and meta-analyses of school-based interventions, which form the primary evidence base; longitudinal cohort studies; and cross-sectional or out-of-school observational studies, which are used only as supplementary evidence and cannot show how school-based interventions work. Meta-analyses and systematic reviews indicate that such interventions produce small but statistically significant improvements in depressive symptoms, anxiety, self-esteem, and social competence, although effects are inconsistent across studies, specific to particular outcomes, and are often absent in unselected samples. Mediation studies, most of them cross-sectional and few embedded in school programmes, converge on self-efficacy, self-esteem, social support, and emotion regulation as candidate pathways, frequently modeled as serial (chain) effects; such models describe statistical mediation and do not establish the mechanisms through which interventions act. Sex, age, socioeconomic status, and the school and family environment plausibly modify these effects, but moderation findings are inconsistent in direction, with stronger associations reported in girls for some outcomes and in boys for others, so recommendations for sex-specific programming remain premature. Substantial disagreement persists regarding the relative efficacy of different intervention modalities, the shape of the dose-response relationship, and whether short-term gains can be sustained, a question that itself conflates maintenance of activity behavior, persistence of psychological benefit, and long-term correlates of sport participation. We argue that methodological limitations, the neglect of social inequality, the near-absence of mediation testing within trials, and the largely untested potential of emerging delivery models constitute priority directions for a field that could yet deliver scalable and sustainable mental health benefits for young people.