A growing body of research demonstrates that school climate is associated with adolescent mental health. Much of the extant literature have focused on identifying risk factors for adverse mental health outcomes, whereas much less work has been done examining factors that are important to foster adolescents’ strengths and thereby increase the likelihood of positive outcomes. Moreover, most studies have used a cross-sectional design and therefore been unable to evaluate possible mechanisms linking school support to mental health outcomes. In a sample of 288 adolescents with and without attention-deficit/hyperactivity disorder (ADHD), the current prospective longitudinal study examined whether school support is indirectly associated with depressive symptoms, as well as academic competence, self-worth, and self-efficacy, via three types of academic motivation: intrinsic motivation, extrinsic motivation, and amotivation. We also explored the potential moderating role of ADHD status. Results demonstrated that higher school support in 8th grade had a direct effect on lower depressive symptoms, increased academic competence, and increased self-efficacy in 10th grade. Further, school climate predicted subsequent outcomes via academic amotivation in 9th grade. ADHD group status did not moderate the indirect effect of any outcomes. Findings underscore school environment as important in understanding pathways to depression and positive self-perceptions in adolescence, in part via amotivation for academic work and activities.