Family-school partnership interventions can effectively address behavioral concerns in students (T. E. Smith et al., 2020). However, little is known about factors impacting the successful delivery of behavioral strategies by parents and teachers in the context of such interventions, particularly for adolescent populations. This study used a mixed methods design to examine the treatment integrity of behavior support plans (BSPs) for adolescents in middle school. Parents and teachers of eight middle school students at risk for emotional or behavioral difficulties engaged in conjoint behavioral consultation to develop and implement BSPs. Participants completed measures of stress and student behavioral severity prior to implementing BSPs, and treatment integrity was monitored during implementation. Open-ended survey questions gathered qualitative implementation information from parents and teachers at the conclusion of the intervention. Quantitative findings indicated negative correlations between parenting stress and days of implementation (r = -0.93) and between teacher perceptions of student behavioral severity and adherence to the BSP (r = -0.81). Qualitative findings identified barriers to implementation (e.g., time and scheduling, intensity of student behavior); facilitators of implementation (e.g., alignment with values and practices, collaboration across participants); and parent endorsement of changes to parenting practices. Quantitative and qualitative results were in partial agreement regarding the relationship between student behavioral severity and implementation, and the triangulated results expanded our understanding of barriers and facilitators to implementing BSPs. Limitations and implications for science and practice are discussed.