Emotional (i.e., internalizing) and behavior-related (i.e., externalizing) mental health problems are widespread among adolescents, but how and to what extent do they inhibit students’ success is school? This study examines the relation between mental health problems and academic achievement at the end of compulsory education by linking a population-based Norwegian health survey on 12-15 year olds with administrative data (N = 4,202). The analyses reveal a robust negative influence of attention problems and conduct problems on average grade achievement, while the role of internalizing problems is limited. Comparing students’ teacher-assigned grades and anonymously graded exams, I also find evidence suggesting negative teacher-bias in grading related to attention problems and conduct problems. Overall, the findings show that behavior-related mental health problems adversely affects students’ academic achievement and suggest that this in part reflects a social response in teachers’ grading decisions.