Challenging classroom behavior can interfere with learning. Fortunately known, positive, and proactive approaches to behavior management can improve student responding. Class-Wide Function-related Intervention Teams (CW-FIT) have led to improvement in student behavior across elementary and middle school contexts. However, little is known of the impact of the intervention on high school student behavior. This study evaluated CW-FIT's utility in improving high school student and high school teacher behavior in a co-taught learning environment. A single-subject withdrawal design was used to evaluate improvements in on-task behavior for 14 high school students in one co-taught classroom. The impact on praise and reprimand statements of two high school teachers was also assessed. The findings showed improvement to student and teacher behavior and sustainability of the intervention. Further, teachers and students expressed satisfaction with the intervention and teachers maintained high levels of implementation fidelity. Limitations of the evaluation and areas for future research are presented.