Despite great advancements in the development of evidence-based treatments (EBTs) for youth mental health problems, few EBTs have been adopted by or successfully implemented in schools. This is of concern, as schools are the most common entry point for youth mental health services. Modular psychotherapies may be a particularly good fit for the school context given their flexible nature. This study examined the experiences of school counselors implementing of a modular therapy. School counselors (n = 20) were recruited from a larger randomized controlled effectiveness trial in five school districts. Counselors participated in semi-structured interviews, and content analysis was used to elucidate barriers and facilitators to successful implementation of the protocol. Barriers and facilitators fell into four broad categories, consistent with the Consolidated Framework for Implementation Research: (1) intervention components, (2) school setting, (3) school counselors, and (4) study-specific processes. School counselors generally found the flexible nature of the protocol to be a good fit for their students and emphasized the benefits of training and ongoing consultation. Counselors highlighted many logistical barriers specific to the school counseling setting (e.g., lack of time, space, and competing job demands). Findings underscore the need for the development, testing, and implementation of brief and flexible mental health treatments that are tailored through school stakeholder engagement.