I would figure out what she enjoys, and build her learning off of those topics. It's great that she does well in math, but is that what she really enjoys? At her age I would make sure she is engaged in what she is being taught, especially with supplementary education. She will have plenty of time for differential equations in college, there is not a need to do that in high school (or earlier).
Also, remember that changing to a more 'challenging' school can have as many negative effects as it does positive. I have met many people who became so self absorbed in IQ, alma mater, GPA, etc that they end up loosing sight of the big picture. There is a lot to learn from traditional schools beyond the classroom, those other kids are probably a good representation of the future general population and if your daughter ends up being a physician, therapist, lawyer, police officer, firefighter, teacher, business person, or just about any other career she will be dealing with these same people for the rest of her life.
