jclem, post up the info.
Make it to where in order to enter the giveaway, they must show OS proof they have donated to said charity. Minimum amount could be whatever you set, and for every multiple over, it gives another chance to enter in the giveaway thread. Then, random number generator it and Viola. No money has ever touched anybody's hands except the charity.
I'll see if I can figure out how to make it work if no one wants to do the raffle. The school does have a website where people can donate with a credit/debit card, but I don't want to exclude the cash/check donations either. I'll get something figured out if you guys don't. I've been thinking about it for awhile anyway.