The Time of the Ghost - Diana Wynne Jones

There's been an accident!
Something's wrong!
She doesn't know who she is, and doesn't know why she's invisibly floating through the buildings and grounds of a half-remembered boarding school. Then, to her horror, she encounters the ancient evil that four peculiar sisters have unwittingly woken — and learns she is their only hope against a deadly danger.
A ghost, uncertain of her identity, watches the four Melford sisters hatch a plan to get their parents' attention and slowly becomes aware of the danger from a supernatural power unleashed by the girls and their friends from the boys boarding school run by the Melfords.