I don't think I am qualified to answer that, really.
The general advice seems to be if you are going to do something that will require short, intense burst you may find your levels rising.
If you do gentle steady things like walking, it should not go up any further.
I don't know to be entirely honest, though, and I think it is individual a lot too.
Could you make some sort of agreement with her school that if levels are high, she can do a walk or gymnastics exercise instead of the thing she dislikes? May take the pressure/excitement out of the exercise, allow her to get a bit of movement, not encourage her to try and avoid it, and probably mean her levels would be more steady than otherwise?
I think to be honest, I would ask here diabetes team for advice also. There's a component of youngster, a component of diabetes and a component of school rules as well as she may have gotten a scare, so she is trying to prevent a repeat. I think I'd phone them for advice, that's what they're there for after all, too.