Past research suggests that the relationship between health and schooling is axiomatic.Physical activity, including sport participation, putatively facilitates school performance. However, the direction of this link lacks clarity. We examine the mutual links between sport and classroom engagement in 452 boys and 514 girls from ages 6 to 12 years. Participants are from the Quebec Longitudinal Study of Child Development, a prospective-longitudinal birth cohort. First, trajectories of classroom engagement from ages 6 to 10 years, assessed by teachers, were generated using latent class analysis. Second, analyses of covariance (ANCOVAs) compared leisure time physical activity, self-reported by children at age 12 years, across trajectories of classroom engagement. Third, ANCOVAs compared classroom engagement, measured by teachers at age 12 years, across trajectories of extracurricular sport between ages 6 to 10 years. We identified two classroom engagement trajectories: ‘High’ (77%) and ‘Moderate’ (23%). For girls, being in the ‘High’ trajectory predicted significantly higher levels of physical activity (F(1, 966) = 5.21, p < .05). For boys, being in the ‘Consistent participation’ extracurricular sport trajectory predicted significantly higher levels of classroom engagement (F(1, 966) = 6.29, p < .05). Our analyses controlled for pre-existing individual and family factors. Our findings suggest that sport participation and engaged classroom behavior positively influence each other during childhood. They support the pertinence of investing financial resources in youth intervention so that children can develop their potential both in sporting contexts and in the classroom to foster optimal growth and development.