Notes on a Scandal by Zoë Heller

When the new teacher first arrives, Barbara immediately senses that this woman will be different from the rest of her staff-room colleagues. But Barbara is not the only one to feel that Sheba is special, and before too long Sheba is involved in an illicit affair with a pupil.
Barbara finds the relationship abhorrent, of course, but she is the only adult in whom Sheba can properly confide. So when the liaison is found out and Sheba's life falls apart, Barbara is there...