In this paper, we describe a large-scale pedagogical setting involving groups of students from different profiles gathered around a real-scale design project (re-shaping the waiting room of a mental health center). Ergonomics students' main task is to analyze the needs and real activities of end-users; high school students' task is to propose inspiring design tracks; interior architecture students' task is to produce the design project; industrial drafting students' task is to realize execution plans while construction students' task is to implement the project on site. This communication more precisely focuses on the role of ergonomists in the setup, describing their intervention and the practical and pedagogical innovations put in place to help them face the various challenges encountered during the project, namely dealing with the temporal constraints of the intervention, documenting and observing a sensitive situation and involving end-users to make them heart of the design process. The paper concludes with feedback gathered from the different stakeholders.