Try lunging her under saddle before you ride; make the lunge work as much a part of the session as riding her. Let her work off some of that energy and focus on you before you get on. Also, if she's starts bucking on the lunge, you can reprimand her without the danger of falling off. If you are consistent, she will learn rather quickly that when the tack goes on it is work time, not play time.
If the problem persists, it might be a balance issue; cantering with a person onboard can be hard for a youngster to figure out sometimes. Could be a saddle fit issue. Maybe she's sensitive and protesting strong aids or a too-strong seat. Since I can't see you ride her, its obviously hard to tell. Good luck :)