Here are all the details I remember. I read the book in the 90's. The cover may have involved an archway?
The title could have been "Seven/Nine gates to _____." but honestly not sure.
The story centers on a girl who finds a portal to another realm (One was located under a bridge?). She meets a troubled boy from school and realizes that the world is his creation to escape an abusive father. I remember that they found a pocket watch that had significant meaning to him and was used as the magic weapon to destroy the villain (his father?).
There may also have been something to do with him bleeding and a rose. Vague I know, but it has been eating at me for two decades.