A true LAN doesn't use consoles....

Just tell everyone to bring a laptop and then find a router or two for the connection or if you have wifi aceess, use the school's network.

Couple of games that are easy to get and not too hard on system requiremments:

Warcraft 3: Frozen Throne (If melee isn't your style, get Dota)
Counterstrike 1.6 or Source (King of all LAN FPS imo)
Starcraft: Brood War (You can just spawn everything)