This paper advances in the understanding of motivation in terms of flow in groups from a physiological perspective. We use wearable devices to monitor the heart rate variation during a set of sessions of face-to-face STEAM project-based learning. By using Action Research with mixed-methods design, we observed a set of 28 students in real-world settings during 18 classes and used both customized and commercial tools to analyze data retrieved. Based on the cognitive absorption and motivation obtained from EduFlow-scale-based physiological data, we propose mathematical models to predict the Flow that a group will experience in a teaching-learning session. Our preliminary results may challenge the central axiom of Flow Theory, while clarifies the balance hypothesis.