AbstractBackgroundEducational difficulties are an important potential influence on both the onset and course of children's conduct problems. This study evaluated the association between school failure and children's conduct problems in Brazil, a context with high rates of both conditions, using both observational and genetic approaches.MethodsProspective, population‐based, birth cohort study in Pelotas city, Brazil. Parents reported on conduct problems four times between ages 4–15 years, and group‐based trajectory analysis was used to classify 3469 children into trajectories of childhood‐limited, early‐onset persistent, adolescence‐onset, or low conduct problems. School failure was measured as having repeated a school grade up to age 11, and a polygenic risk score (PRS) predicting educational attainment was calculated. Multinomial adjusted regression models were used to estimate the association between school failure (observational measure and the PRS) and conduct problem trajectories. To consider possible variation in effects of school failure by social context, interactions were tested with family income and school environment (using both observational and PRS methods).ResultsChildren repeating a school grade had increased odds of being on to childhood‐limited (OR: 1.57; 95% CI 1.21; 2.03), adolescence‐onset (OR: 1.96; 95% CI 1.39; 2.75), or early‐onset persistent trajectory (OR: 2.99; 95% CI 1.85; 4.83), compared to the low conduct problem trajectory. School failure also predicted increased risk for early‐onset persistent problems versus the childhood‐limited problems (OR: 1.91; 95% CI 1.17; 3.09). Using a genetic PRS approach, similar findings were observed. Associations varied according to the school environment: school failure had larger effects on children in better school environments.ConclusionSchool performance, whether measured in terms of repeating school grades or genetic susceptibility, was consistently associated with trajectories of child conduct problems into mid‐adolescence. We also found a larger association for children in better school environments.