Windows 2003 Server and multiple logons

I am using Windows 2003 server and connected to 400 or so windows 2000 and XP Pro pc’s. Is there any way in the server to prevent multiple logins with the same username? This is a school environment and I am trying to prevent a student from signing on to more then 1 pc with his/her username and password at the same time.
From what I can tell, a 3rd part application might have to be used.
Thank You