Most high schools I've been in have a PA system that's designed for speech, not music, so it would not be suitable for drums. My recommendation would be to find out what the rest of the group (bass player, keyboard player) is using. You can use their wattage to gauge where you need to be to keep up with them... and add 10-20% for headroom.
A TransActive Drummer or two would probably be appropriate as a monitor system in the pit for you and the band. For the house, assuming it's a ~1,000-seat room, something in the 300-500 watt range is probably where you need to be. You could either look into a small PA system (two boxes and a sub) or look at full-range keyboard amps, preferably with an 18" woofer.
As far as getting audio out of the computer, you'll want to use a firewire interface, and not a USB interface or the built-in headphone jack. Firewire is far lower in latency and much more reliable in a setup like this.
Hope this helps,
Dan