Well, Richard, this is strange. Most complaints are about users not receiving emails of posts when they are subscribed to a forum.
Maybe someone else can help out here. For now, go to each teacher's profile and change the "Email digest type" field to "Subjects (daily email with subjects only)". That way they should receive only one email each day with all the posted subject headings.
Are you totally certain the emails they receive are coming from the courses and system you are working on? I know that sounds like a crazy question, but I can't figure out why they continue to receive email notifications if they are not subscribed to the forum.
Check out several of the emails the teachers receive - click on the link that goes back to the forum post and see where you end up.
The only forum that forces subscription is the News Forum, but students cannot post to that.
This is puzzling - keep posting here so we can find a solution.