BACKGROUND: Adolescent mental health difficulties are rising in the United Kingdom (UK), yet school-based provision is limited. Nature-based Programmes (NbPs) are increasingly promoted as alternative or supplementary interventions for Mental Health and Wellbeing (MHWB), but evidence in secondary schools is under-explored. AIM: This scoping review investigates NbPs in secondary school contexts; identifies reported outcomes for mental health, wellbeing, and learning; and assesses evidence quality for NbPs in secondary-school provision. Although this review synthesises international evidence, it is framed by growing interest in NbPs in UK secondary schools and aims to inform UK policy and practice. METHODS: We followed PRISMA-ScR guidelines, searching five databases and grey literature. Eligible studies reported school-based NbPs for students aged 10-18. Screening, data extraction, and appraisal (using MMAT) were undertaken with policymakers, educators, and young people. RESULTS: From 17,723 records, 22 studies were included. NbPs encompassed school gardening, outdoor education, conservation, forest pedagogy, recreation in nature, and passive nature engagement. Most NbPs were delivered weekly on schoolgrounds or in local greenspaces. Reported MHWB outcomes included reduced anxiety and stress, improved self-esteem and resilience, and enhanced prosocial behaviour, school connectedness, and nature connectedness. Evidence for academic outcomes was mixed, with modest gains in science engagement and arithmetic but little evidence of broader academic benefits. Study quality was moderate, with small samples, limited control groups, and heterogeneity in outcome metrics. CONCLUSION: NbPs show promise for supporting adolescent MHWB in UK secondary schools, with school and nature connectedness emerging as potential mediators. However, evidence for cognitive and academic gains remains limited.