I agree with the rest of the commenters, ask her what she wants. Smart kids don't stop being smart just because their classes are easier, and by the time she hits college things will likely even out. I was in a similar situation but really liked my friends, so I stayed where I was and I'm really happy I did. I have lifelong friends and things have really worked out for me.

That being said, if she's entirely indifferent, and you're not worried about burnout, it's always good to help make sure she's intellectually engaged. I was very bored for most of elementary and middle school, and I do regret that somewhat.

Finally, I will say that my true passions I pursued outside of school. If she has interests, make them accessible to her whether they be sports, chess, programming, dance knitting, science, etc. Find clubs and books.