I wrote a little app quite a while ago that ran when a user logged in or out and then updated a MySQL database. I never got around to making any web pages though, and never made it into a full blown app. My intention was to write another app that would display all the computers in the room (after the teacher selected one) and display who was logged on it, but I never got around to it. If you're interested then email me at firstname.lastname@example.org. You're more than welcome to try it out and see if it meets your needs.
quirky but simple
our login script contains the cmd
echo %time% %username% > \\server\logs$\users\%computername%.txt
if your computers have room prefixes on their names then each file gives the information:
room, computer, logged on user
it would be easy to make an asp page that searches for the correct files for a room prefix given in the url and displays the information.
my scripts here aren't helpful as they're tied into isa server and do a load of wierd isa stuff that isn't relevant.