Collaboration between different professions is thought to be important for mental health work in schools. However, high-quality collaboration is often difficult to achieve in practice, and there is little research available to inform strategies for strengthening collaboration. This study analyzes the effects of the research and development project "School as an arena for child and adolescent mental health," aimed at developing practices for promoting students' mental health in Norway. We collected questionnaire data from all professions participating in collaboration teams in the four participating municipalities (n = 40) and 27 comparison municipalities (n = 197). The main findings indicate that the project initiative positively affected innovation practices in collaboration. We discuss the results in light of the design of the project and field data collected over a 4-year period.