The aim of the study has been to examine the impact of diagnosed internalising and externalising behavioural problems on educational attainment. We used a fixed-effect model on rich individual longitudinal register data. The sample consisted of five full cohorts of adolescents (N=242,542). The analyses suggest that compared to their healthy peers, boys and girls with externalising problems have respectively 38 and 40 percentage points lower probability of completing upper secondary school. The comparable numbers for internalising problems are 29 percentage points for boys and 26 percentage points for girls. With regard to the likelihood of attending higher education, for those that completed secondary school, the results show a negative but much smaller impact of mental health disorders than the case was in the analysis of upper secondary school completion.