Controlling Internet access

I want to be able to restrict internet access to students in a classroom while allowing others in other areas access.
Internet access is via router. The server, and other users are linked via switch. The classroom has its switch which is in turn linked to the switch with server and other users. See crude diagram below. How can I restrict the Internet to the classroom while still allowing it access the server?

Router
|
S w i t c h
| | |
____________| | |
| | |
Switch | |
| | |
| | |
Classroom Users Server1(PDC)