Create a group for your classes and have all students who are enrolled in this particular class join the group so that you can send group notices about the course, homework reminders, etc. You might forbid students to use this group to send group messages to their peers. You might, however, also allow them to use it in some cases (saying which) so that they can help each other or chat out side class.
Do not use this group to add all people who are interested in your classes. You can create a separate group for that purpose.
Creating a group in Second Life costs L$100 and at least one other person besides you needs to join the group within 48 hours otherwise the group will be cancelled with no refund and you will not be able to use that name again.
Update: Also remember that you can only join 25 group with one avatar. You might, therefore, want to reuse groups with new classes.