Correct me if my terminology is wrong but:
I want to set up a Blog to send my comments and challenges to my students and allow them to reply. From what I understand, a Blog is be a one-way comment with two way replies. I pose the questions, students reply to that post and can reply to other replies but cannot start a new post at the same level as mine.
I can set up for all to start new posts and all can reply, or no one can post anything except me. The only option I can see is in the quick-edit for that page to allow or disallow comments. This makes it all or none.
Elsewhere in the web site, I have a forum allowing a free for all.