A couple of ideas to share:
From Sara Schoepke: I have been promoting blogging with students in my district as a means for
teachers to get students writing (Common Core) as well as give students a way to
use higher level thinking in all content areas. Here are some of the resources
I have created to help my teachers implement blogging in their classrooms.
Kidblog.org is perfect for blogging/discussions. I see many teachers use it for literature discussion groups - setting up a page for each book with a small group of students collaborating and chatting.
With Kidblog you can enroll in classes with a Google Apps for Education account, too, which makes it simple for teacher set up.