halbert5150

Personally I use vmwares free ESXi for this. ESXi plays nice with either windows or nix desktops. Setup a centos Vm, create 12 unique logons / vnc profiles. Then have the students install a vnc vlient to access the server.

I know this is an older post but I hope you still stumble back on this. I had the exact same issue and I found it was my firewall. My firewall was stripping unknown header data and I made a rule that circumvented the problem. I cant tell you exactly what the header item was that was stripped as my firewall vender came out with an update that also fixed the issue so I was able to delete the rule. Hope this helps.