This study presents findings from a 58 high-school group-randomized controlled trial testing the effectiveness of training in a multitiered system of supports for behavior (MTSS-B) framework, which was leveraged to reduce students’ risk for emotional and behavior disorders. The trial tested the impact of MTSS-B, which included (a) training in the broader MTSS-B framework that went beyond the existing Tier 1 (school-wide PBIS) training offered by the state; (b) project-provided coaching and technical assistance supports; and (c) integration and training in evidence-based behavioral or social-emotional programs at Tiers 2 and/or 3. We reported effects of MTSS-B on implementation of positive behavior supports across all three tiers using the Schoolwide Evaluation Tool (SET) and Individual Student Systems Evaluation Tool (ISSET), as well as on external observations of teachers’ use of classroom management strategies. Results indicated significant effects on multiple SET subscales and significant reductions in teachers’ use of reactive behavior management.