Me and two friends are doing the graduation trip thing as well, and we're 19, 18 and 4 months, and 18 and 1 month. My mom (the one I'm learning the tricks of the trade from) made the reservation for us in my name, and the three of us are going to share a one bedroom at Kidani.
I've also stayed at OKW with a bunch of high schoolers and a handful of adult chaperones. We were performing in the Candlelight Processional, and I can count on one hand the number of eighteen year olds we had in the group, but they put three to four of us in a room without an adult, and I don't think any resort personnel complained to our director.