Mental disorders may for various reasons impair educational attainment, and with far-reaching consequences given the impact of education on subsequent employment, social life, life choices and even health and mortality. This register-based study addresses trends in educational attainment among Finnish adolescents aged 13–17 with mental disorders severe enough to necessitate inpatient treatment between 1980 and 2010. Our subjects (N = 14,435), followed up until the end of 2014, were at greater risk of discontinuing education beyond compulsory comprehensive school or of lower educational attainment than their age-peers in general population. Only 50.0% had completed any post-comprehensive education compared to 84.9% in same-aged general population. Those at highest risk were males and those with organic, intellectual disabilities and developmental, externalizing disorders or schizophrenia group diagnoses. Despite improvements in adolescent psychiatric care, school welfare services and pedagogical support, risks have remained high. Greater effort in psychiatric treatment, school welfare and pedagogy are needed to combat this severe inequality.