This participatory action research aimed to understand teachers' experiences and the development process of collaborative expertise in a school well-being program implemented in Finnish comprehensive schools. Teachers from two schools participated in a school well-being program for one and a half school years. During the program, they reflected on their workplace practices, processes and actions for promoting school well-being during the program. The research included focus group interviews at the beginning and end of the program. The thematic analysis revealed that key elements for collaborative expertise are courage to accept uncertainty, understanding school culture as a constant and complex development process, and educational leadership. These elements enable a reflective, broad-minded and resilient school culture, which is essential for developing teachers' collaborative expertise in school well-being. In conclusion, teacher educators, educational researchers and education policymakers are called on to reorganise initial teacher training, the induction phase of teachers and in-service training from the perspectives of expertise and organisational studies. Additional process-oriented, continuous development and novel methods and practices are also needed to enhance well-being in the whole school community.