Disableing internet access to specific users in Win 2k server….

I am attempting to disable access to the internet to specific users. The problem is I work at a school where the users (students) are using the same machine. The students are divided into morning and afternoon classes some of the students use the internet the appropriatly but some do not need nor deserve the internet. I have tried disableing the machine they student sits at but it is troublesome.

Can anyone tell me a way to disable the users by account?