Block IP addresses from students

I am supporting a small school that is trying to be more secure with their network. They are running Server 2003 and Windows XP clients in a domain set up with static IP addresses. Some of the students have figured out how to put a static IP on their own laptops, IPAD’s etc. The school asked if I could secure the IP addresses so they can’t use them. They only want the students using the schools equipment but can’t stop them from bringing things from home. So they want to block them from internet access. There is no firewall, only a DSL modem and the server. I am drawing a blank on how to stop them from internet access if they put in a static IP themselves. Any ideas would be appreciated.