I doubt if it is a high school kid, but it could be
I think it probably is a high school kid who probably has been rejected, abused, has no social skills, is bitter and dangerously violent. An adult with this kind of repressed anger would probably already be in jail by now. This is a youngster who probably has never had to answer for his actions and isn't close to anyone.
He may not be close to anyone, but I'd bet he wanted to be close to this girl. Maybe he was rejected. It's definitely someone this girl knows, maybe as a pest or a jerk. He is also probably calculating and smart from the note he sent. As were the Columbine killers.