Stop unknown pcs connecting to network

HI all,

I manage a small school network running Win2003 server in a domain. There are lots of students around and at times I have found them plugging in their laptops right into an open outlet, or even worse, disconnecting an unused PC and using that port.

My question, is there a way to stop DHCP from giving out addresses to unknown machines or perhaps classifying known machines in a way that only they get IP addresses?

Thanks

Nate