This cluster randomized controlled trial evaluated the efficacy of the CHAMPS classroom management program on the social behavioral and academic outcomes of a large diverse sample of middle school students within an urban context. Participants included 102 teachers and 1,450 students in sixth to eighth grade. Two-level hierarchical linear models (HLM) were conducted to examine the overall treatment effects on student behavior and academic outcomes. In addition, mediation analyses examined a hypothesized putative mechanism for observed academic outcomes. Findings indicated that CHAMPS improved teacher ratings of student concentration problems (d = -0.18) and classwork completion (d = 0.18), observed student time-on-task (d = 0.16), and student scores on broad English (d = 0.14), and math problem solving (d = 0.17) academic achievement tests. Null effects were observed for student prosocial and disruptive behaviors and self-regulation skills as well as reading comprehension and broad math achievement performance. Main effects on the English achievement test scores were partially mediated by student improvements in observed time-on-task. Practical significance of the findings and implications for schools and policymakers are discussed. [This is the online first version of an article published in "Journal of Educational Psychology" (ISSN 0022-0663).]