Belgian primary schools very rarely adopt an outdoor learning approach. Yet most geography and science teacher trainers involved in pre-service teacher training see the environment around schools as a resource on which to develop learning in all disciplines, and a place to develop environmental and eco-citizenship education. Training teachers to follow this vision is hard, especially now with a generation of students who have a self-confined lifestyle, lack interest in walking and have almost no connection with nature. In the Haute École Libre mosane (Liège, Belgium), an alternative strategy based on a service-learning approach was tested for two years during a whole term: volunteer students followed an eco-traineeship in a non-formal organization, for which they had to design and complete an environmental education project. The main results of this action research are presented here.