Background: Transitioning into adolescence while simultaneously facing greater academic demands as the level of education increases often entails both academic challenges and general declines in students' school-related well-being. Still, however, relatively little is known about the causal relationship between students' academic well-being (i.e., school engagement and burnout) and their performance during the adolescent years. Aims: This study examined longitudinal relations between adolescents' mathematics performance, school engagement and burnout (exhaustion, cynicism and inadequacy) across lower secondary education. Sample: Data came from a longitudinal research project, following Finnish lower secondary school (grades 7--9) students (N = 1131) over 4 years (2016-2019). Methods: Students completed standardized mathematics tests and self-report measures of school engagement and burnout at four time points, twice within both 7th and 9th grade. A random intercept cross-lagged panel model (RI-CLPM) was used to examine pathways between engagement, burnout and mathematics performance over time. Results: Higher mathematics performance increased students' engagement and lowered their exhaustion and cynicism over time, whereas both engagement and exhaustion predicted higher performance. Negative relations were also found from inadequacy and cynicism on students' mathematics performance. Furthermore, school burnout predicted engagement both positively (from exhaustion) and negatively (from cynicism and inadequacy) within and between the school years, whereas engagement only predicted cynicism and inadequacy negatively within 7th grade. Conclusions: Findings suggest that the overall relation between students' mathematics performance, engagement and burnout is rather reciprocal, but also, that the relations become more prominent over time, demonstrating the importance of supporting both learning and well-being in school.