Forum

I administer several courses that are always open for enrolment. Students have one year to complete the course upon enrolment. All students have typically stayed enrolled in the course (neither I nor they have been unenrolling) after one year, so often times students will respond to a post that was written by a student no longer active in the course. This is sort of a "dead end," since there will not be opportunity for that thread to
continue. I'd like to encourage more interaction within forums - in other words, students communicating with others who are currently actively working on the course. Any ideas on how to do this?