Background: There are known health disparities in adolescent substance treatment access and engagement. The purpose of this project is to compare outcomes from school- and clinic-based substance treatment and to evaluate if providing school-based substance treatment reduces disparities in treatment access and engagement. Method: This quality improvement retrospective chart review compares baseline and outcome data for adolescents accessing school-based (n = 531) and clinic-based (n = 523) substance treatment in a natural quasi-experimental study with nonequivalent control group design. Baseline demographic and clinical measures include age, sex, ethnicity, race, and clinical diagnoses. Outcome measures include the number of sessions completed, proportion reaching a week of self-reported abstinence, and proportion providing a negative urine drug screen. Results: Compared to the clinic-based sample, the school-based sample includes more female (47.65% vs 26.77%) and Hispanic/Latinx (59.89% vs 46.46%) adolescents. The school-based group has a similar proportion reaching a negative urine drug screen (31.84% vs 28.83%, p = 0.5259) or a week of abstinence (43.15% vs 41.03%, p = 0.6718) as the clinic-based sample. There are significant differences in total session completion over a period of 16 weeks between school-based and clinic-based adolescents. In multivariable analyses, there was a significant interaction effect of race/ethnicity by location on the number of sessions completed. Conclusion: Providing school-based substance treatment increases access to care and treatment engagement for female, African American, and Hispanic/Latinx adolescents without diminishing outcomes.