At present in Moodle 2+ there is no quick way for a teacher to open up a 1-1 dialogue with all students in the class, like a private forum, dialogue etc.
*Groups can do it of course. But the overhead of renaming groups for everyone with their names means this is not a 'quick' way. I did find this tracker item from December: http://tracker.moodle.org/browse/MDL-21000
*Dialogue module can do it, but it is not in the core. http://tracker.moodle.org/browse/CONTRIB-2059
*Assignment can do it of course: but this has the wrong hegemony, with the idea of 'black hat' assessment rather than formative conversations.
I have thought a bit before creating this as an issue. It is the functionality I think Moodle needs, HOW it is done I'm not too worried about. I'm sure I have seen other tracker issues over the months to try to address this, and as I find them I'll comment.