The research used in this thesis considers the effects of a public policy that aims at improving the French educational system by mobilizing concepts and approaches that come from both pragmatic sociology and sociology of translation. This improvement project is embodied in elementary school in the “Plus de maîtres que de classes” plan, which prescribes collective work. Indeed, the plan enables the assignation of an additional teacher where pupils have persisting schooling difficulties. The enquiry on teachers’ work effects is based on an ethnographic approach, with a prolonged presence in the field that has enabled us to create monographs for three contrasted socio-demographic schools. We examined modalities and effects of the plan to understand how teachers’ collective work within a single classroom is implemented, structured and how it affects the job itself. Results of that research show that collective work can be a lever for the professionalization of teachers as well as for the evolution of their professionalism through an intensification of exchanges related to student learning, a reinforcement of collective work, and the different perspectives on each other’s work, etc. regardless of the many ways one can express, perceive and implement modalities of this plan.Teachers explain their way of proceeding according to several principles of justice (equality, efficiency, tradition, etc.) that they mobilize in any given situation, resulting in a professional world dominated by pragmatism and based on the hybridity of principles of justice. New pedagogical organizations enabled by this plan have created a new division of educational tasks that questions teacher’s professional identity as well as the emergence of a new occupation, the additional teacher.Finally, this plan shows a professionalization process and a professionalism development through a collective work under certain conditions: techniques of interpretation and operators’ adaptation to the injunctions as well as to the specifics of their environment, whether or not to adopt shared principles of justice according to what they consider fair for the common welfare and the weight of certain socio-demographic criteria on the profession in favor of collective work. On the contrary, collective work can also lead to the teachers feeling that they are being de-professionalized because of a reduced autonomy and also because of the evolution of their profession.