UNLABELLED: Rising public concern about comprehensive child and adolescent well-being has led to the development of school-based interventions with the potential for high-reaching and effective support. While some interventions have shown effectiveness, limited understanding exists regarding how social inequalities are considered and evidenced in such interventions. This study examines how social inequalities are considered in universal school-based interventions and their potential to affect inequalities through differential effects. A systematic review following the PRISMA protocol was conducted using the following databases: PubMed, Web of Science, CINAHL, Scopus, ProQuest and APA PsycArticles. Studies published between 2014 and 2023 were included. Screening and data extraction were conducted independently by two researchers. Of 10 028 initial articles, 44 were included in the final analysis. These studies primarily involved physical activity and mindfulness interventions in schools. Despite many studies including information regarding students' social backgrounds, such as socioeconomic position and immigrant background, the analysis of differential intervention effects among demographic groups was limited and mostly based on sex. Most differential effect analyses showed no significant differences based on social background, and no clear differences were found based on intervention type. While some universal school-based interventions show promise in reducing social inequalities in students' well-being, more empirical research is needed to explicitly target these questions. This review highlights the critical need for comprehensive intervention studies to consider and report relevant dimensions of social background and their interactions with intervention effects. TRIAL REGISTRATION: PROSPERO
registration no. CRD42023423448.