Promoting physical activity (PA) among young people and combating a sedentary lifestyle are public health priorities. Beyond social marketing campaigns and the dissemination of information, numerous grassroots initiatives have been developed over the past several years to promote physical activity. For practitioners and funders, the question arises regarding the selection and implementation of interventions, in a context increasingly driven by the pursuit of effectiveness and efficiency. Consequently, the identification and subsequent scaling up of scientifically validated interventions are major challenges in supporting stakeholders. The results of the Icaps study (Intervention with Middle School Students Focused on Physical Activity and Sedentary Behavior) demonstrated that it is possible to increase physical activity and reduce sedentary behavior, with a beneficial effect on young people’s health. This intervention is part of a socio-ecological approach that takes three areas into account: the young person, their social support, and their physical and organizational environment. Following a pilot deployment phase, a second phase began in 2019. The objective is to support the implementation of Icaps projects through a national operator, based on the Icaps approach and in collaboration with the school community. Thus, during the 2022–2023 school year, in 13 participating schools, more than 1,300 of the 1,567 targeted youth achieved the objectives of an Icaps project (i.e., +1 hour of PA per week and a 20-minute reduction in daily screen time). There was also a significant increase in the number of young people meeting national physical activity recommendations between the start and end of the school year (from 8% to 26%). Four categories of actions emerged within the schools that enabled effective implementation of the approach. Finally, these results were achieved thanks to partnerships among the various key stakeholders.