What is the name of this book?

I don't remember much. It was about this high school girl in a Catholic school I believe. She is followed around by a ghost who's she has fallen in love with. She talks with one of the monks/teachers/pastors at her school who can see him too, to go for it. She meets a man who can go into this Alterniate universe where she can join the ghost and live with him. Eventually, she realizes this can't work and I remember at the end of the book the ghosts gives in and they are together.