when I had it working with the version you have installed, I had a mixed network (one knoppix and one windows machine). Never tried it with two knoppix machines (would like to, but I doubt I'll get the time).

an alternative to KDE Desktop Sharing is to use the latest beta of RealVNC 4.0 which has the capability of sharing a desktop. To do this though, you would need to install it on every machine... I've no idea how you auto-update 15-30 Debian desptops at a time :( (a case of building your own .deb and getting your students to install the package in class?)

Once installed, you would set up the VNC server with a passowrd, and then (I imagine) write a script so that VNC is started each time X is started (such scripts can be found googling). You would need to find a way to stop your students from killing VNC (saying that I don't know if they would be able to kill VNC without killing X at the same time...).

I'd like to help you out more, but I don't have that much time and I'm too inexeperienced I'm afraid. What I would suggest though is subscribing to the RealVNC mailing list (I get the digest), and checking out the archives. Haven't been able to read the digests for a while now, but I remember VNC class-room questions popping up quite regularly.

If I see anything/try anything, I'll post here and/or PM you.

Have fun!