Scary Mary by S.A. Hunter

Mary has always been different. She'd like to be normal, but being able to hear ghosts means she'll never be like everyone else. She starts her junior year of high school hoping to be left alone, but Cyrus Asher is new and doesn't know or seem to care that she's an outcast. They start hanging out and all is well until she goes over to his house. Cy's house is haunted, and not by Caspar, the friendly ghost. 

But it's not the ghost that ruins the evening. That honor belongs to Vicky "The Hickey" Nelson with her borrowed Ouija board and stuck-up friends. They make Mary so angry that she uses the ghost to freak out everybody, Cy included. He orders her out, and Mary thinks she's lost whatever chance she had with him. But there's still the ghost to deal with. He's mean, nasty, and possibly homicidal. She has to get rid of him or Cy and his family could be hurt. Or worse.