This paper presents a study on the role of the teacher in computer-supported class group activities. We discuss various teacher tools that support this role. In the reported studies the students are engaged in group activities through networked computers. Typically they use a two-space collaboration tool. One shared space used for jointly producing a diagrammatic representation (concept map or other form of diagram), and one for text based communication. The group activities have a time span of typical classes: i.e. a few minutes to a few hours. In this context, we focus on the study of typical teacher actions and used representations and on the support that the tools used provided to the teacher for supervision of the class and group activities.