This case study provides insight into the ways an interprofessional team developed collaborative competencies to enhance school‐based mental health support for students with a dual diagnosis of intellectual disabilities and mental health illness. The team was anchored by the school division coordinator, two resource teachers, a social worker, and a psychologist. It is also a site for preservice nursing, social work, recreational therapy, and psychology students who were completing their senior practicum in their respective programs. The purpose of this study is to develop a deeper understanding of how preservice and in‐service professionals experience interprofessional collaboration and competency development as a means to support students with dual diagnosis. Interviews were conducted with 21, preservice practicum students and in‐service professionals who participated on the team. Data were recorded, transcribed verbatim and manually aggregated, coded, and themed using two complementary strategies, constant comparison method and initial and focused coding. The Canadian Interprofessional Health Collaborative (CIHC) (Gilbert, ) framework helped illuminate the fundamental components, that is, the competencies, needed to be an effective collaborator. The findings suggest that four of the six CIHC competencies, role clarification, student centeredness, communication and team functioning emerged as core competencies. The practicum students described their experience on the team as an enriching way to develop specialized knowledge, skills, and attitudes with students with dual diagnosis and they continue to serve as advocates in their current positions. Reciprocally, the permanent structure of the interprofessional practicum offered a unique professional development strategy for in‐service professionals. This study queries the need to refine the CIHC model for school‐based mental health settings. It is suggested that the model would be enhanced by a greater emphasis on culturally responsible practice and a greater integration of inclusive education competencies for professionals who work with students who have dual diagnosis in schools.