BACKGROUND: One in seven adolescents globally are affected by mental health conditions, yet only a minority receive professional help. School-based mental health services have been endorsed as an effective way to increase access to mental health support for people at risk, or currently presenting with mental health conditions, throughout adolescence. Despite this, low treatment utilisation prevails, therefore the aim of this review is to contribute insights into the processes related to adolescents' accessing and engaging with essential targeted mental health support within schools. METHODS: This systematic review extracted qualitative, quantitative and mixed-methods data to determine what processes affect adolescents seeking help from targeted school-based mental health services (TSMS). Searches were conducted in EMBASE, Medline, PsycINFO, CINAHL, ERIC, Web of Science, in addition to manual searching and expert consultations. Data were synthesised following guidelines for thematic synthesis and narrative style synthesis. RESULTS: The search resulted in 22 articles reflecting 16 studies with participant sample sizes ranging from n = 7 to n = 122. Three main themes were identified: 'access-related factors', 'concerns related to stigma', and 'the school setting'. These findings elucidate how help-seeking processes are variable and can be facilitated or hindered depending on the circumstance. We identified disparities with certain groups, such as those from low-socio economic or ethnic minority backgrounds, facing more acute challenges in seeking help. Help-seeking behaviours were notably influenced by concerns related to peers; an influence further accentuated by minority groups given the importance of social recognition. Conflicting academic schedules significantly contribute to characterising treatment barriers. CONCLUSIONS: The findings of this review ought to guide the delivery and development of TSMS to facilitate access and promote help-seeking behaviours. Particularly, given the evidence gaps identified in the field, future studies should prioritise investigating TSMS in low- and middle-income settings and through quantitative methodologies. REGISTRATION: The protocol for this systematic review was registered on PROSPERO (ID CRD42023406824).