|I always get suspicious when someone say they bought a computer and cant get past a log in screen requiring a password. I also think it a bit strange that a school would sell a computer with its own software still on it. I would have thought they would have wiped it first.|
Assuming this is legitimate and some body at the school released a computer without first wiping it, the only thing you can do is reinstall the OS. It is probably configured to connect to a domain controller and in order to change that you need to get to the desktop first.
Then again it might be a disk less workstation in which case you are going to need to install a hard disk.