In Ontario, as elsewhere in Canada, a discourse of prevention aimed specifically at young people permeates the fields of health and social services. This discourse encourages the use of evidence-based approaches, including psychotherapy, to intervene with children and adolescents whose behaviors are perceived to be problematic or at risk. The use of these approaches is often prescribed to prevent mental health problems in young people. In this perspective, this article analyzes the processes that lead social workers in French-language secondary schools in Ontario to apply intervention techniques to manage the behaviors of students. Based on a theoretical and critical approach that combines the notion of bio-power with labeling theories, the microsociological analysis focusses on the discourses of social workers and the roles of various actors in the recent implementation in schools of the Ontario government’s mental health strategy.