Shared Internet Access Software??

I have been asked to put a small network into a local small school with a limited budget. The easiest (and cheapest) means of getting internet to the classroom is via their current dialup. However I would like individual control of the modem by each room - they should be able to switch the modem on and dial up from the room rather than sending a student to the office to liven the modem.

Does anyone know of cheap (maybe even free ) software to do this such as Wingate. We intend running Win2K on the server and XP Pro on the workstations.