A young teen girl goes to a school for soldiers, gets tricked into murdering her (male) friend, carries his ghost up a ladder.

I read it between 2000 and 2007. I think i got it from the library, so it could have been any age but likely a few years old and still in print at that time. I don't remember much from it, i remember it being a very dark book, but that's my only clear memory from in it.