Background and Objective: Supporting children in developing health skills goes, according to research, through a comprehensive approach to health promotion. The Alliance for health project aims to enact and assess the impact of a comprehensive approach to health promotion at the school and community levels. This intersectoral project involves regional education authorities, regional public health agency, local policy makers, researchers, and local health promotion operators. Methods: This project involved 48 French primary schools and 47 associated municipalities in the AURA region randomly selected. From September 2019 to June 2022, the intervention was implemented. Primary school teachers were provided training, support, and resources to develop a health-promoting environment in school according to the Health-Promoting Schools framework. Meanwhile, school deputies and territorial agents involved in extracurricular activities were provided training, support, and resources in the same aim. Intersectoral working sessions were promoted. To analyze implementation outcomes, semi-structured interviews were conducted in Spring 2022. Results: This qualitative study included 11 district pedagogical advisors, 10 school principals, 9 school deputies, 2 extracurricular agents, 1 person in charge of after-school programs and 1 health promotion project manager. Schools’ perspective. Identified facilitators were a constant and available referral, institutional support and dedicated time, clear aim and issues from the beginning, community support, multi-school training sessions, contextualized support. Barriers were COVID-pandemic, lack of community support, lack of time. Community perspective. Identified facilitators were health promotion perceived legitimacy, collective motivation, regular and effective working sessions. Barriers were perceived methodological incompetence, administrative burden, lack of time. Conclusions: Collective work dynamics, practices’ exchanges, health promotion professional identity are essential implementation components. Lack of time is perceived as a structural barrier. Communities pointed out perceived legitimacy and competences as corner-stones of their engagement.