Background: Well-being is a key aspect of children's education, yet measurement issues have limited studies in early primary school. Aims: The current 12-month longitudinal study assesses the temporal stability of child- and parent-reported school well-being and examines developmental links with academic self-concept and parent-rated prosocial behaviour. Sample(s): We tracked a sample of 206 children across the transition from the first (T1) to the second (T2) year of primary school (T1 child M[subscript age] = 5.3, SD = 0.46, 54.3% girls) and gathered ratings of well-being, prosocial behaviour and academic self-concept at both timepoints. Methods: We used cross-lagged analyses to investigate developmental links between these three constructs. Results: Parent and child reports of children's well-being showed similar temporal stability and converged over time, such that informants' reports showed a modest but significant correlation at T2. Girls reported greater well-being than boys at both timepoints and received higher parental ratings of well-being than boys at T2. For both girls and boys, associations between the constructs were asymmetric: early well-being predicted later self-concept and prosocial behaviour, but the reciprocal associations were not significant. Conclusions: These findings support the validity of young children's self-reported well-being, highlight the early onset of gender differences in school well-being and demonstrate that early well-being heralds later prosocial behaviour and positive academic self-concepts.