Oh, so you're in the east. I got confused as to what school we were talking about when you mentioned Western Orange county. Now that we know you go to Storm King, lets talk about power.
One of the BEST things you can do to keep power requirements down is to avoid CRTs like the plague. Most people have LCDs and laptops which is a good thing, but I'd make it a rule that no CRTs are allowed.
Your best bet is to go and test the power layout of the room you are going to be using. Make your own map and walk around the gym and see where the power outlets are. Since it's a gym, there is the possibility of having outlets in the floors as well as the walls, and maybe even coming down from the ceiling (tho that can be harder to access obviously).
Once you know where all the outlets are you need to figure out what outlets attach to what BREAKERS. Any good sized LAN party WILL pop a breaker or two (dozen). You need to know where the breakers are so you can 1) reset them and 2) know how to layout your power.
Go to Home depot and get one of
THESE. Or if you are like most students and broke, get a portable lamp. Then get a buddy with a walkie talkie (if the breaker is in another room) or cell phone and start testing outlets to see which breaker they are on. Start with one outlet and start flipping switches until the light goes out. Note which switch runs that outlet on the map (they should have numbers next to them in the panel). Move to the next outlet, wash rinse repeat. The tester is handy for making sure the outlet is wired properly as well.
Since you are at a school I would suggest talking to the janitor/custodian before doing this to make sure you don't turn off the headmasters' hot water mid-shower.

Note on your map what breaker controls what outlets. Some older building might even require you to do a little of your own labelling on the breakers. Masking tape is your friend. You could even go so far (on or near the event) to label the outlets with colored masking tape to indicate which outlets are on what circuit.
Once you know what goes where, make note of how many AMPS the circuits you have available to you are rated for. This will be your primary means of knowing how much power you have to play with. 20-30 amp circuits are much more useful than 10-15 amps for multiple computers. Figure on approximately one computer per amp AT LEAST. You might need 2 amps for higher powered computers.
Ultimately the only way to know is to play around and test shit out.
I'd lay things out such that PCs are on a different circuit than your Consoles and TVs.
Cornwall ain't too far from me. I'm sure for some a free LAN party I could be convinced to come over and help out with setup and stuff. And for future LANs open to outsiders we could easily get a dozen or so people from here to come up and join you.