I'm obviously not a high school athlete (a bit too old for that!), but I am an athlete. I am always training for something (well, I've been injured this year, so haven't been training a lot this year)... but on previous cruises, even though I do have access to the fitness center, I just find I like being outdoors more rather than cooped up in the fitness center on the ship. For me, I do all of my running on deck 4, I also do strength training on deck 4, and I bring all my own stuff (I bring resistance bands, stretch cords, etc... stuff that is small, compact, very easy to pack). I find it pretty easy to maintain my physical fitness and strength outside of the fitness center on the ship... coming from Washington State where it's cloudy and rainy a lot, any opportunity I can have to workout outdoors, especially with the warm breeze, I take!
That said, I cannot comment on someone under the age of 18 (or whatever the age limit is for the fitness center), but I just wanted to let you know from my experience, I've been able to maintain my fitness level very well on all the cruises I've done on deck 4 (and also never using the elevator and always using the stairs works the quads and hamstrings well!! Especially if you start on deck 3 and go up to deck 10 and back down again several times in a row for part of a workout).
I'm sure she has very specific things she needs to do being a gymnist, but I just wonder if she talks to her coach if there are things he/she could recommend she can do outdoors with resistance bands, etc. And, I have no idea, but I even wonder if you could go and talk with the person in charge of the fitness center on the ship and see if you can borrow a stability ball and a couple medicine balls for the week (seems like they'd have extras in a storage room somewhere??) and keep them in your room and she can use those, too, in your room?? I have no idea... but could be worth asking!!
Good luck! I totally understand about wanting to keep up your physical fitness routine on vacation. I hope she can figure out something!!
Kathy