Family-school partnership interventions can effectively address behavioral concerns in students (Smith et al., 2020). However, little is known about factors impacting 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 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 a family-school partnership intervention (conjoint behavioral consultation; CBC) 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 were used to gather qualitative implementation information from parent and teachers in writing following conclusion of the intervention. Quantitative findings included 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 specific 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 outcomes of participation (i.e., changes to parenting practices). Quantitative and qualitative results were in partial agreement with respect to the relationship between student behavioral severity and implementation, and expanded descriptions of barriers and facilitators from either dataset individually. Limitations of the current study and implications for science and practice are discussed. [This paper was published in "School Psychology" v38 n6 p385-398 2023.]