Class reunion committees are non-profit organizations, usually. To protect yourself you should have at least one other person, (and two others would be better) involved. Keep accurate records. You should have a checking account for paying bills. Your canceled checks will help prove, if anyone asks, that all of the money you collected went towards the reunion.
Ours runs a little surplus (not "profit") every five years. We don't take it out, we don't spend it on reunion committee parties. We keep it until the next reunion, when it goes for postage for those who still don't have e-mail and such. We have to print flyers and mail them out before any money comes in. After our 50th we will give the balance to the old school scholarship fund.
Whether you register as a non-profit or not is your call. As long as you can prove you didn't make a profit, you should be fine. "Profit" here means you collected $20,000, spent $19,000 and transfered the $1,000 to your personal account, the way you would if you had a business. Keep the $1,000 in the account opened in the name of "Central High Class of 19xx".