Stoping Multiple Logins in w2k domain

I admin a school network and the administration wants to stop students from sharing accounts by preventing multiple simultaneous logins.

The network is a Windows 2000 Advanced Server with active directory.
The client computers are a mix of
XP Pro (The majority)
w2k pro
Win 98. (The win 98 computers are being phased out and can be ignored when answering this question)

Does anyone have any ideas on how to:
1. Prevent the multiple logins via group policy or additional software approach
2. Any techniques which can aid me in finding and reporting these violations of use policy without resorting to #1.

Thanks,
Tom Augstein