We present a real-time interactive tagging system called SynTag for collaborative learning in lectures. The system allows students accessing the web-based interface to tag an ongoing lecture. The collective tagging outcomes are instantly visualized with a dynamic timeline chart to illustrate the assessment of all students. With SynTag, students not only engage in a lecture by the interactive tagging function, but also cooperate with each other to contextualize the lecture. According to the outcomes of students' collaborative tagging activities, the critical spatio-temporal segments of a lecture video are automatically generated as the wave-shape timeline chart. Through the peaks of the wave-shape timeline chart, the valuable video chips of the lecture can be easily highlighted and recognized. In the end of this paper, we demonstrate how the SynTag system contributes to the collaborative learning in undergraduate classes through quantitative and qualitative studies. Finally, we conclude with a discussion based on our evaluation for the improvements and potentials of our system in future works.