You actually dont need Windows or BIOS to change the time, if the machines have floppy drives just make a boot disk at home and take it in, then type "date" at the command prompt.
If that doesnt do it, use the fact you can browse the hard disk to edit the win.ini file (inside the windows folder) to change the default shell from explorer.exe to either progman.exe or taskman.exe and reboot, you will now have access to a run dialogue. Actually doing it this way is just a handy trick to know as it bypasses the registry bootstrap so it's good for removing virus', and if you've not access to the hard disk you can do all this from DOS. You could just run either program from within the windows folder, and both have a Run dialogue.
I'm not sure what application changes the time on XP, but have a look for anything with date or time in the filename, i'm on a Vista machine at the moment and it's different so cant do this. In fact i've never bothered to figure out how to hack Vista like this as so far i've not needed too.
If your means of accessing the hard disk gives you search functionality you should be home and dry.
EDIT: If they dont have floppy drives you can make a bootable CD from the bootdisk, bootable CD's are just CD's with an imagefile of a bootable floppy. Your burning software should give you all you need to do this.