This is an old post, I know, but since it's been left without an answer I thought I'd add my own
I believe newer Moodle releases have the option to only subscribe to the discussion one has added (or replied to) in a forum.
All you need to do is to switch the forum subscription mode from "forced" or "auto" (unless you prefer the 'op-out' finctionality) to optional.
Also, if you already have students automatically auto-subscribed to the entire forum, clear the list of subscribed users.
Upgrading the student role would only be necessary if you want to use the News forum as the helpdesk forum. If you add an extra forum for the purpose, students are normally allowed to add posts etc.
Oh well, probably all this is a non-issue now ;) Cheers.