Simple Time Sychronization

I work at a small school with a bell system based on a clock that I am not able to change (don't ask). Therefore, for convenience sake, I'd like to have every computer on the campus to have its time synchronized with our domain controller according to a time that I set. I know how to set up the GP to synch with time.windows.com, but how do I setup our domain controller as the NTP?

Domain controller: Windows Server 2003 R2
Clients: Windows XP