Background: Poor academic (eg, "I am a bad student") and behavioral (eg, "I am a troublemaker") self-concepts are strongly linked to adolescent substance use. Social networks likely influence self-concept. However, little is understood about the role teachers and athletic coaches play in shaping both academic and behavioral self-concepts. Methods: We analyzed cross-sectional surveys of 929 9th-12th grade low-income minority adolescents in Los Angeles assessing self-concept, social networks, and 30-day use of alcohol, marijuana and other drugs. We performed generalized estimating equations, accounting for clustering at the school level and controlling for family and peer influences and contextual factors. We also tested whether self-concept-mediated associations between relationships with teachers or coaches and 30-day substance use. Results: More perceived teacher support was associated with lower odds of marijuana and other drug use and better academic and behavioral self-concepts. Behavioral self-concept mediated the associations between teacher support and substance use. Conclusions: By facilitating relationships with adults and improving teachers' capacity to build supportive environments, schools may positively shape how adolescents see themselves, which might help reduce adolescent substance use.