The largest synthesis of its kind, this meta-analysis draws on 125 longitudinal studies to establish that student engagement declines consistently across adolescence — a finding that holds across behavioural, emotional and cognitive dimensions. The decline is small but statistically significant and is steepest in early adolescence, particularly at the point of school transition from primary to secondary. Studies from Europe and North America show the sharpest drops. The paper confirms the predictions of stage-environment fit theory: as students develop, school environments increasingly fail to meet their psychological needs for autonomy and competence. For educators and policymakers, the key implication is clear — transitions in early adolescence are a critical window for intervention.