If a forum is set to be restricted to only users with the teacher role, the students can't see the forum. That works fine. But if the forum is set to "Auto subscription", then all the course participants gets on the subscribers list.
This is quite surprising for the teacher...
The subscribers list should match the participants with the right role to the forum.
Please vote fore this bug at: http://tracker.moodle.org/browse/MDL-35027
BR Per