I went on schoolies last year underage. I took an i.d with me that was a girls who looked like me. I had it for the first three nights and they were definetly my best up there, at the end of the 3rd night the last place me and my group went the guard took it off me so we headed home. I was heaps pissed off at first but I had other underage friends up there so I met up with them and still had a good time for the rest of my time up there.
So basically if you can get your hands on someones i.d that just looks a little similar to you, do it! The guards arn't going to charge you, all they can do is "I dont think this is you you're not coming in buddy" and they take it off you and give you a card that says where you can pick it up the next day if you can provide other proof that its yours.
Having said that, there are heaps of people partying in the middle of the street on Cavil Ave and theres also lots of people down on the beach just hanging out so even without an i.d you have a really good time.