Our communication focuses on "constellations", an institutional in-service training scheme for schoolteachers deployed in France since 2018 (Villani et al., 2018). The aim of this program is to improve students' academic results in mathematics and French, which recent assessments (DEEP, 2023; PISA; TIMMS) have revealed to be fragile. The novelty, changes, values, and processes attesting to the innovative dimensions of this scheme (Cros, 2009) are linked to the ways in which teachers are engaged in their training. These modalities are inspired by collaborative research (Desgagné and Bednarz, 2005), promoting reflexive and collaborative analysis embedded in teachers' actual practices. They therefore contrast with previous top-down training methods, which international surveys have shown to be ineffective in terms of teacher development (Malet et al., 2021). Our aim is to analyze the effective implementation of constellations in the Aix-Marseille Academy, regarding disciplinary contexts (mathematics, French) as well as geographical contexts (urban, rural, priority education). This will enable us to identify specific criteria for the quality and effectiveness of this training (Council of the European Union, 2009). The methodology combines several scales of analysis relating to the management and conduct of training and its impact on classroom teaching practices. The main results point to the context-dependent adjustments the trainers made to the prescribed system. These concern the structure of the constellation process, the negotiation of work objects, and the involvement of teachers in the design/analysis of the situations they experienced. Furthermore, the evaluation of the "constellations" program raises questions about the quality and efficiency of its nationwide deployment regarding its impact on the professional skills of trainers and primary school teachers, and more broadly on the evolution of their professional development.