Software key loggers generally don't work unless you already have the admin password. All processes not run as a system service in an NT based computer are shut down after logging off of the station. Therefore, it would not be active after logging back into the computer. This also includes VB scripts.
Hardware key loggers are better as long as the keyboard plug is not in plain sight behind the computer. If a student is under a desk and messing around with the computer, that's going to draw suspicion.
Lastly, most professional System Admins use remote login when accessing a remote computer. They also tend to change that password every two weeks or so.
As my fellow spoofians have state earlier, this is not a wise move. Don't mess with something that can get you into trouble or expelled. If you get caught, and you most likely will, they can press charges and then the police will get involved.
Besides, what is so important that you would need this information?