Worldweavers: Gift of the Unmage by Alma Alexander

Thea lives in a world much like our own, except for the fact that everyone does magic. Everyone but her, and since she is a fabled seventh child of two seventh children, her lack of magic is particularly tough to bear. Her father send her packing through a portal to a Native American teacher to try to find her magical gifts. Thea spends the first half of the book learning off in the spirit world, and the second half saving her world from some nameless menace that doesn't even show up until the last third of the story. A rambling, inconsistent book -- NAY.