I have a sneaking suspicion this is a Fear Street book, but I'm having no luck pinning it down from the lists online. The protagonist is a teenage girl, who starts seeing a ghost in her bedroom. Nothing terribly saucy happens, but she gets to talking to him, and eventually starts being interested in him romantically. I want to say he's dressed like a stereotypical greaser, although that might be influenced by the book art, which I remember as showing the two of them in her bedroom with him being slightly transparent. He can't remember who he is, or how he died. I don't remember if there were any visible wounds on his spirit body that might have clued her in. Eventually, through some set of circumstances, she runs into him outside of her bedroom, except he's alive, and doesn't recognize her, leading to a climax where she realizes that somehow his ghost traveled back in time yet, and he hasn't died yet. He gets into a fight, and a knife gets drawn. I don't remember if she manages to intervene before he gets stabbed, and whether he survives.
I believe I read this somewhere in Kentucky in the mid 1990s in a paperback book, maybe 150ish pages.