EMediatheque is a collaborative, synchronous e-learning platform developed by eLycée S.A.S, providing various collaborative tools such as shared whiteboards, instant messaging, shared web browsing, synchronous multimedia viewing, etc., on top of a multi-party video-conferencing system and a highly configurable UI. This platform allows students and teachers to meet and communicate in virtual classrooms, and perform collaborative pedagogical activities involving the use of various sorts of resources (audio, video, Web, etc.) and document co-construction. The originality of eMediatheque is that it was designed to support the tracing of meaningful user interactions with the platform (actions perceivable by the user), allowing to build a rich and contextual usage history that we refer to as the primary trace. It also comprises a real-time trace visualization system that transforms and abstracts the primary trace (with respect to the ongoing activity) into a more understandable form to the user, in order to provide a reflexive perception of its own activity. These visualized traces support interactive parameterization (filtering, merging, etc.), may be shared and confronted, and provide shortcuts for generic operations (undo/redo, replay, etc). In showing its own trace to the user, we expect to provide a reflexive dimension to the activity in order to improve the pedagogical process. In this paper, we discuss practical issues of the use of traces in such a context: how to build a trace-enabled collaborative platform; how to set up interactive trace visualization. We also discuss the potential benefits regarding reflexivity and awareness, and the main difficulties in building up a visualization that makes sense for the student or the teacher in the context of his or her pedagogical activity. The discussion is illustrated by a concrete application: a simple activity where two students co-translate a cartoon and can use their traces to be aware of their past actions.