You are off topic for the forum you posted to, which is dedicated to Microsoft Security Esentials Beta
You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
The complicated solution, though more secure, is to purchase, install and configure a proxy server or firewall. The easy solution, easily overcome by your students, involves editing the hosts file on each PC.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator