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Background Schools play a prominent role in the delivery and evaluation of public health initiatives. However, participant recruitment and retention are established challenges, with evidence suggesting differential participation across population subgroups, resulting in bias in the evidence base. As the importance of inclusion and diversity within research gains increasing recognition, exploring representation within the current evidence base could help inform action. We therefore aimed to identify subgroups of young people systematically underrepresented in National Institute for Health Research (NIHR)-funded school-based prevention research in the UK.Methods A systematic search of the NIHR Journals Library aimed to identify publications of UK-based school-based health prevention trials, published between 2004 and 2024 and reporting on a primary health outcome for children or adolescents (aged 4–19 years). This search focused on the NIHR Journals Library because NIHR is the UK’s largest funder of high-quality health intervention research and its peer-reviewed reports are typically highly detailed.Two independent reviewers screened studies for inclusion, extracted data into a bespoke extraction table, and undertook quality assessment using the validated Effective Public Health Practice Project Quality Assessment Tool. The NIHR Innovations in Clinical Trial Design and Delivery for the Under-served (INCLUDE) framework, modified in collaboration with secondary school students and staff, was used to identify population sub-groups. Findings were synthesised using Synthesis Without Meta-analysis. Where possible proportional representation ratios (PRR) were calculated, adjusted for sample size, comparing research samples to national population estimates.Results The search identified 107 unique references; 22 trial reports were included. Results showed that male/female gender (binary) were proportionally well-represented at baseline (PRR=0.98) and follow-up (PRR=0.95). Participants from global ethnic majorities were moderately well represented at baseline (PRR=0.78) but were more likely to be lost to follow-up (p=0.04). Samples included participants from all four UK nations, although 85% of participants lived in England. Within England, <1% of participants lived in the North. Insufficient data was available to assess participation by socio-economic position, English language, special educational needs, absenteeism, school-exclusion, and health status.Conclusion UK-based evidence on school-based prevention is not representative of the UK population and improvements are needed to better capture the ethnic and geographic diversity of the UK. To address research inclusion standardised, frequent, and uniform reporting of sub-group baseline and follow-up characteristics at both student and school levels is required. Future research should use targeted, inclusive recruitment and retention strategies to increase research diversity.