Background Schools are an increasingly popular setting to conduct and implement public health research and initiatives. However, recruiting diverse samples of adolescents can be challenging. With growing concerns surrounding representation of research samples, understanding who is systematically missing from the evidence base andwhy is important. Working with students, school staff, and a youth worker from diverse backgrounds, this systematic review aimed to explore what encourages/deters adolescent participation in school-based health research. Secondary aims were to compare experiences and perspectives for systematically missing or ‘under-researched groups’ to those from general population samples.

Methods Search terms were developed through analysis of key publications, with decisions made in consultation with stakeholders. The strategy included terms relating to educational settings, participation, qualitative methods, and focused on mental health/obesity as two research priority areas. Relevant studies were identified through systematic searches of 8 electronic bibliographic databases. Studies were included if they reported on a) qualitative data, b) exploring participation experiences of, c) adolescents (age 11-19) in, d) school-based obesity and/or mental health research, e) conducted in mainstream educational settings, and f) within high-income countries. Additionally, studies had to be published in English Language within peer-reviewed academic journals. Appraisal of included research was conducted using the Critical Appraisal Skills Programme qualitative checklist. A thematic synthesis of qualitative data was undertaken using Thomas & Harden’s (2008) approach.

Results In total, 4,112 studies were screened, of which 11 were eligible for inclusion. Results from our thematic synthesis highlighted complex and varied factors influence participation. However, we identified 2 major themes: 1) research engagement and enrolment and 2) sustained and purposeful participation. Factors influencing participation at both surface and deeper levels were subtly different. To initially engage/enrol adolescents within research, balancing perceived investment with incentives/support received was important. Whereas, to promote sustained and purposeful participation researchers needed to empower participants, evoke feelings of comfort and safety, minimise burden, and evidence a tangible impact/benefit from research participation. Factors which influenced under-researched and general population samples’ decisions to participate are similar, but sometimes these manifested in different ways. Those from under-researched groups were more likely to emphasise altruistic motives, describe stigma, and/or environments where health was not prioritised.

Conclusion Our study distils the key components required to establish best practice for study recruitment and engagement in school-based health research. This knowledge could help inform future inclusive participation strategies to conduct high-impact research that connects and engages all population subgroups.