Middle school is a cornerstone for success in high school. However, cognitive, emotional, and behavioral engagement of adolescents often decline as they transition from elementary to middle school, resulting in lower school grades and more challenging behaviors, especially for adolescents with or at risk for emotional and behavioral disorders (EBD). Class-wide function-related intervention teams (CW-FIT), a classroom management program based on school-wide positive behavior support, has been found to be effective in elementary general and special education classrooms. The present study, a randomized control trial of CW-FIT for middle schools (CW-FIT MS), evaluated the effects on class-wide on-task behavior, as well as the on-task and disruptive behaviors of students identified as at risk for EBD. Participants included 629 students (70 identified as at risk) enrolled in 28 middle school classrooms across five Title I schools representing diverse geographic, ethnic, and socioeconomic areas. The study also addressed research questions regarding effects of the intervention on teacher praise and reprimands, as well as teacher and student social validity. Results suggested that CW-FIT MS was associated with improvements in class-wide and at-risk student on-task behavior, student disruptions, teacher praise, and teacher reprimand rates. Both teachers and students reported the intervention to be socially valid. Study limitations and areas for future research are addressed. [This paper will be published in "Journal of Behavioral Education."]