In Wallonia, special efforts are needed in order to respect the international recommendations on physical activity in youth. School is definitely considered as a central pillar in the promotion of an active lifestyle. The aim of this study was to analyze the implementation process of daily activity breaks called ‘Physical Activity Pause at School’ (PAPS) in 6 primary schools of the Liege area. 23 classroom teachers were involved. They had to propose at least one PAPS/day during the 15-16 school year. Diary, questionnaires, focus groups and interviews were used for data collecting. Two thirds of the subjects continued until the end. The videos were not systematically used. Four teachers’ profiles were identified: Innovators, Invested, Careful, and Disengaged. 12 factors facilitating or limitating the involvement of the teachers have been analyzed. The study comes to a model of concrete actions to respect in order to increase the chances of success in the implementation of such project.