When I set up a forum for separate groups, the following two issues appear:

(1) Non-editing teachers in one group receive email notifications from all groups, including groups they are not a member off.
(2) In the forum activity summary, the posting count and unread count reflect all postings across all groups, rather than those for their own group

Clearly (1) is a complete breakdown of privacy and has forced me to set subscription to "no subscriptions allowed" until this is resolved.