In this paper, we investigate how social tagging could be used in Education as a support for learning processes. We first summarize the results of recent works on the effect of the use of social tagging for knowledge building and learning. We show that few educational tag-based systems intentionally use tagging to enhance learning. We then describe a Tag-based Collaborative System (TaCS), meant for supporting social and collaborative learning thanks to tagging, and detail the learning processes expected by the use of the system. We conducted an exploratory study to observe (1) the evolution of the students' tags as an indicator of the learning of new concepts and (2) the evolution of the tags assigned to documents as an indicator of the learning of new conceptual relations. The results show that the students make their tags and their relations to documents evolve, mainly due to two activities: the comparison of individual and collective tag clouds and the negotiation for an agreement on a common tag cloud in the groups.