On a forum with groups enabled (separate or visible -doesn't matter), a teacher (= leagacy role editingteacher) is not able to reply to messages by members of a group unless the teacher is also member of that group.

mod/forum:replypost allows replying, but groups seem to overrule this. mod/forum:editanypost allows you to edit all posts, even those made by members of groups you are not a member. mod/forum:replynews doesn't affect this, and no other forum permission seems appropriate. This leads to two unintuitive situations:
 - a teacher may be able to edit a post but not reply to it
 - a teacher must add himself to all groups to be able to use forums in a way that he would expect to.

"Workaround" is simple, add the teacher to all groups. It might even be that this is like it is by design, but I think no, it's a bug more likely.

I tested this on 1.8.2+, and it has been reported on 1.9.