This article seeks to contribute to the reflection around the training of future designers regarding teamwork. Collaboration, teamwork and negotiations are common everyday interactions that are now known to contribute positively to the design process. This article builds on a theoretical model that was initially proposed as a Ph.D. thesis contribution: the zone of proximal development for learning teamwork skills. The objective of this article is to share the first implementation of the model in a real educational situation and seize this opportunity to develop it further. The framework of assessment for learning and the concept of the zone of proximal development guide this research. The data collection is based on the observations of teamwork during a design seminar. The analysis is structured around the theoretical definitions of the model's components and in situ observations of the participants. This process guides us to share indicators allowing to confirm if a level or set of skills are achieved by a student. Plus, we propose a set of related teaching guidance to use when skills are identified as underdeveloped or to develop. Through this work, we wish to discuss how design students can be better equipped to enter the profession and play a significant role as team players.