The Song of the Lioness Quartet

The Song of the Lioness Quartet is a series by Tamora Pierce featuring a strong female protagonist, Alanna.  In the kingdom where Alanna lives, she is expected to grow up to become a proper young lady in the palace court and her bother, Thom, is expected to train to become a knight.  Neither want to pursue the path fate has for them and so they hatch a plan to trade places and assume each other's future.  The four books in the series follow Alanna's journey as she grows up, gains skills and wins hearts.

This series would be of interest to middle school aged students, possibly especially girls, although there is action and adventure enough for all kids to enjoy.