Mental health problems--and depression in particular--have been rising internationally. The link between poor mental health and poor educational outcomes is particularly interesting in the case of the UK which has a low international ranking both on measures of child wellbeing and the probability of early drop-out from the labour market and education. We study this issue using a large longitudinal study of a recent cohort of teenagers in England. We use the General Health Questionnaire to derive measures of poor mental health. We find a large negative association between mental health problems and educational outcomes--where we consider examination results before leaving compulsory education and the probability of being "not in education, employment or training" at a young age. The association is large even after including a very rich set of controls. Results are stronger for girls and also vary according to the different components of the mental health measure. We also explore the potential role of intermediary mechanisms (truancy and risky behaviours). (Contains 14 tables and 11 footnotes.)