Context: the number of twelve to seventeen-year-olds who are overweight or obese has more than doubled over the past twenty-five years. To date, few studies have documented collaborative practices in activities aimed at promoting healthy eating and physical activity in schools, although collaboration is a key element in health promotion. Objectives: to identify the determinants that influence collaborative practices in secondary schools, and to define the role of the school nurse in these practices. Method: a descriptive multiple case study was conducted in three Quebec secondary schools. The methods used were: focus group; interviews (21); observation (5); and document review (3). Qualitative analysis was conducted by more than one researcher. Results: determinants of collaborative practices relate to the individual, the team, and the organization. The time needed to adapt to a role is a determinant that influences the contribution a newly appointed school nurse can make to collaborative practices in a school setting. Conclusion: understanding the determinants that influence collaborative practices paves the way to establishing the conditions conducive to various stakeholders becoming involved in promoting the health of young people.