A Certain Slant of Light (Light, #1) by Laura Whitcomb

In the class of the high school English teacher she has been haunting, Helen feels them: for the first time in 130 years, human eyes are looking at her. They belong to a boy, a boy who has not seemed remarkable until now. And Helen–terrified, but intrigued–is drawn to him. The fact that he is in a body and she is not presents this unlikely couple with their first challenge. But as the lovers struggle to find a way to be together, they begin to discover the secrets of their former lives and of the young people they come to possess.