There are $5 lockers at the front of the park that you can rent for the entire day. Someone mentioned that there are also lockers near the Rip Saw Falls exit, but those are temporary. The teens could bring a change of cloths and store it in the lockers up front, or just leave a change of cloths on the bus.
The only ride that really soaks you is Popeye. Guys can take off their shirts, and if you get in the ride fast enough, you can put your shoes and socks in the middle compartment, but you must be wearing shoes as you enter the ride. The girls can do the same thing if they have a bathing suit on.
You'll eventually dry off after walking around. The key is to keep your shoes and socks dry, otherwise, your feet get waterlogged.
They'll definately have more fun at IOA. If you can plan the trip so you're there at 8am, then do it. The problem with tour bus trips, is that they arrive at 11am, and that's when the park is most busy. My daughter went with her school and they didn't get on many rides. Why? The lines we're 1 hour, and the kids have no way to get an express pass. So they ended up walking around and playing games most of the time.