The transition to middle school is recognized as a period of increased risk for students’ adjustment difficulties across the academic, behavioral, and social domains, underscoring the need for developmentally responsive interventions that address these potential vulnerabilities. This study examined the impact of a multi-component intervention on students’ perceived adjustment during the first year of middle school using a cluster randomized controlled trial design. A total of 24 metropolitan schools (12 intervention), 220 teachers (122 intervention), and 2925 students (1537 intervention) participated. Teachers in intervention schools received training and consultation in the Behavioral, Academic, and Social Engagement (BASE) classroom management model. Significant intervention effects, moderated by students’ gender, race/ethnicity, and economic status, were found for self-reported social anxiety, defiance, willingness to protect peers being bullied, and emotional problems. The results suggest that teachers trained in the BASE model were better able to create supportive classroom contexts during the middle school transition which promoted positive adjustment for particular students. This study stresses the importance of using developmentally appropriate strategies across correlated domains to reduce adjustment difficulties during the transition to middle school.