To examine early school leaving in a longitudinal cohort of all high school students treated for substance use disorder (SUD) and their demographic counterparts in Norway. From the National Patient and National Population Registries, we extracted (a) all high school students born in 1991-1992 who received SUD treatment during 2009-2010 (N = 648; nalcohol = 95, ncannabis = 327, and nother drugs = 226) and (b) their age-and-gender matched counterparts (n = 647). From the National Educational Database, we obtained enrollment and graduation status for these two cohorts throughout the designated school period of 5 years. We estimated the hazards of early school leaving as a function of students' treatment for alcohol, cannabis, and other drug use disorders and other known risk factors. Nine out of 10 adolescents receiving SUD treatment left high school early (89%) compared with one in four (27%) from the matched cohort; 422 (73.5%) of these left high school during or after the treatment year. Multivariate discrete-time models revealed significant and ordered associations between receiving SUD treatment and early school leaving, HRalcohol = 3.09 [1.96, 4.89], HRcannabis = 3.83 [2.64, 5.56], HRother drugs = 5.16 [3.32, 8.03], even after accounting for individual-level (sex, immigrant background, criminal charges, and mental health treatment), family-level (family structure, parental education, and family income), and structural risk factors (municipal size, county employment, and dropout rates). Adolescents receiving SUD treatment remain especially vulnerable for early school leaving. These findings underscore the importance of improving and coordinating health and educational services for youth in SUD treatment. (PsycInfo Database Record (c) 2024 APA, all rights reserved).