While creating a topic, an instructor has the option for "Automatically create multiple topics for groups".  This will automatically create a topic for each group that is selected during this progress.  When group topics are created, the student permissions should be set to none for the new topic, but it inherits the forum permission set which may have the student set as contributor.  Seems like if the instructor uses this option that the topic should remove permissions for students to ensure group permissions are set properly.