This study evaluated the effects of teacher adherence to behavioral treatment on student outcomes. Eighty-four children (ages 7-11) completed a 12-week, collaborative school-home behavioral intervention designed for youth with significant attention-deficit/hyperactivity disorder symptoms and impairment. Teacher adherence was assessed via school mental health provider (SMHP) ratings and daily report card (DRC) implementation. Pre- and posttreatment outcomes included parent and teacher ratings of organizational skills and problem behaviors, observational measures of classroom task engagement and off-task behaviors, and report card standard grades. Using multi-level models to account for clustering by school, teacher adherence rated by SMHPs predicted improvement across teacher- and parent-rated organizational skills, parent-rated problem behaviors, and classroom observations of task engagement and off-task behavior. Higher rates of DRC implementation only predicted improvements in parent-rated organizational skills; percentage of days parents signed the DRC only predicted teacher-rated improvement in organizational skills. Post hoc analyses indicated that teacher adherence and child success with academic targets on the DRC during the first month predicted parent-rated improvement in organizational skills. These results suggest that teacher adherence, particularly when rated by SMHPs, is an important predictor of positive treatment outcomes across both school and home settings. Future research is needed to better understand methods for measuring and optimizing teacher adherence to classroom behavioral interventions.