This study examined the relative influence of adolescents’ supportive relationships with teachers, peers, and parents on trajectories of different dimensions of school engagement from middle to high school and how these associations differed by gender and race or ethnicity. The sample consisted of 1,479 students (52% females, 56% African American). The average growth trajectories of school compliance, participation in extracurricular activities, school identification, and subjective valuing of learning decreased from 7th to 11th grades (mean ages = 12.9 years to 17.2 years). Different sources of social support were not equally important in their impact on school engagement, and the effect of these sources differed by the aspect of engagement studied. For instance, peer social support predicted adolescents’ school compliance more strongly and school identification less strongly than teacher social support.