I am guessing from your post that you live in a rural area with limited local options. It doesn't hurt to look at every possibility even you don't go down that path as it can help clarify what you are looking for and what exists.

Just addressing the question asked, are you thinking of boarding school for now or for when she is older?

Mary Baldwin College in Virgina has the Program for the Exceptionally Gifted starting at age 13. Not sure whether that is of interest.

A quick google search turned up:

http://tip.duke.edu/node/818
The above link is an article on public boarding schools for the gifted. It was produced in 2006 so I am not sure how current the info is. There is a lot of general info in the article from Duke.

There is the Oklahoma School of Science and Maths for year 11 and 12

Throwing the field wide open brings up the following school in the UK which does American University prep from a glance at the website. I am guessing you have UK connections. A friend of mine went to boarding school in Wales from Australia so not completely off the wall.

http://www.wycliffe.co.uk/gifted_and_talented/