Differences in approaches used to deliver school-based mental health and wellbeing programs may be a key consideration for program effectiveness, yet this has not been considered in reviews and meta-analyses to date. Consistent with previous research, this systematic review of 47 studies found that wellbeing programs delivered in schools tended to show small effect sizes for mental health and wellbeing outcomes with effects often not sustained. The review considered the influence of various program factors on effectiveness, and consistent with previous findings, program-based factors like theoretical framework, program length, and session duration did not show reliable patterns for influencing effectiveness. In contrast, pedagogical factors aimed at increasing participant engagement (e.g., using student-centred and active learning approaches), appear more closely linked to improved mental health and wellbeing outcomes. This review has shown that universal programs can be effective in producing better mental health and wellbeing outcomes in secondary school settings when participant engagement is maximised.