Block web specific users and time

Hi All!

I am an instructor in a school network with 40 workstations. We are plugged into 2 1900 switches. One of the instructors wants to block his students from Internet access from 9-12 only. My students would still need access at this time. Any suggestions on a solution to block half of the hosts from Web access at a certain time? We all use the same DNS and DHCP server (Windows 2003). I thought of turning off 1 switch but then they would lose access to the DHCP and DNS which they need for their Intranet/LAN.