Wikipedia: Gobby
“Gobby is a free software collaborative real-time editor available on Windows and Unix-like platforms. It is confirmed to run on the Mac OS X platform, albeit not natively but using Apple’s X11 instead.”
I’ve been using Gobby with colleagues and we find it really useful.
I put together a short Gobby demo video . It wasn’t really planned but it shows how it could be used. It seems like it could be useful for educational purposes but I don’t have much more info at this stage. Its multi-user, you can work on multiple documents, it runs on most of the operating systems we use.
If you have time, its definitely worth a look.