The current study examined the over-time association between externalizing behaviour problems and academic performance during school transition in a cross-lagged design. The main focus was to reveal whether the externalizing behaviour composite and its components separately, including symptoms of CD and ADHD, differ in their relationship with academic performance; and if controlling child- or family-related covariates altered the strength or direction of the relationship. Externalizing behaviour composite was associated with a decrease in academic performance over a 1-year time lag. Academic performance at Grade 6 was associated with low CD symptoms at Grade 7. The effect remained significant when child-related covariates were controlled, but not after controlling family-related covariates. ADHD symptoms systematically had a negative effect on grade 7 GPA, even after child- and family-related covariates were controlled. The results indicate that during early adolescence and school transition, CD and ADHD symptoms differ in their association with academic performance.