I think your daughter and her friends would have a wonderful time. You and your wife should go down and have a vacation for yourselves too! Just not staying at the same place, I'm sure your daughter would be rolling her eyes at that idea, hehe. But I think WDW is a great atmosphere and not as wild as the Jersey Shore. There will be less teenagers there as well, so she's less likely to get into trouble. Nice to hear that you trust your daughter and her friends. But I would always be cautious. Lots of parents say "My son/daughter would never do that". I'm sure the parents of the kids in the hazing incident were saying the same thing...
I was in WDW my senior year as part of a class my senior year. My friends and I had a great time. i admit we did get rowdy at times, but there was no drinking in our group. However, at the hotel we were staying at there was a group of teenagers that got some older guys to buy alcohol for them. Anything is possible, and teenagers will always find a way to get around the rules... I'm 22 so my teenage years are still a vivid memory!!!
If I were you I might call the front desk of the BYC before the trip, and give them a number where you can be reached. So in case the girls do get rowdy or get complaints from other guests, you'll know and can set things straight. I hope she and her friends have a great time! At least you don't have to worry about Cancun just yet...