Okay really need more details. But I do vaguely recall a book similar to this description, unfortunately I think I'm confusing it with parts of Hex Hall by Rachel Hawkins.
First let's see if we are thinking of the same book... Did the main girl character keep hearing the blood voice and eventually trace it back to the third (?) floor of one of the buildings where a previous girl from the school had jumped off and killed herself. Although for some reason I'm thinking the school had previously been a nunnery or asylum where the ghost girl had been locked up because she was evil. And somehow the main girl character gets a key to the room where the girl had been "held?" The ghost girl tries to manipulate the main girl to do something but when she can't she gets another girl? And the ending kinda of sucked because the ghost basically got what it wanted? Sorry I can't remember the name of this book yet but if the plot sounds right I'll keep thinking on it.
