A detective mystery about a mysterious caller calming someone from this guy's school will die and he stalks her trying to protect her

[ExoticAzathtic]

I'm looking for a book I think I read like when I was young in school I think, maybe anywhere from 6-10 years ago, where a male main character somehow gets on a phone call with a man saying that someone who's also in his school but not in his classes (I think, and I think they're like highschool or college) and so he stalks the girl to learn her schedule and whatnot, seeming very creepy, at one point I think he calls the parents pretending to be a telemarketer to learn her address or something but could be wrong, and I think at the end there's a big climatic thing where he meets the guy on the phone but I could be wrong