Yes, You can allow students to run a session without the presence of an instructor by selecting the option "All users join as moderators" under "Roles and Access" when creating the room. This is a common practice for example when creating a study room or group chat as you have described that does not need an assigned facilitator.
However, students cannot create sessions, so you will have to create the initial session. However, to avoid your having to create a new session every time the student groups want to meet, you can set the "End Date" for the session as the end of the semester. Students will still be able to record their sessions every time they log in, and recordings will be published shortly after all users leave the current session. Marking the "End Date" as the end of the semester ( ex. Dec 10, 2015) should allow students to keep using that room for all of their group sessions. Tips: I would recommend that you create a session for each of your groups, ( ex. Group 1, Group 2) . The students can then join the room corresponding to their assigned group number. Note: All students will be able to join ANY room. If you would like to restrict access to just the students in a specific group, you can do this when you create the session under the "Roles and Access" options.