The Time of the Ghost by 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.