Now think about it logically... you'd need legal copies, if the cops / SAFACT should get a 'sniff' of the fact that you're hosting a lan with illegal copies of the game, what do you think will happen?
Your school should just keep trouble away and go legal.
Speak to say, distributors of the game (like Megarom) and ask them for discounts or if they can make a plan or something to HELP.
Or just make sure, each person who comes, bring his OWN LEGAL copy of the game, if they can't, and they bring copies, then it's not YOUR problem, however, if the school hosts the PC's etc... then there can be serious problems.