This happened to my cousin. A guy at her highschool who was a socially inept introvert began following her around between classes in second year. In her third year she realized that he was in all of the same classes as her. Turns out that he found a copy of her preferred class schedule and copied it to a tee.
Not long after she started receiving creepy love letters in her locker. She amassed a good two dozen letters before she took it to the principals office out of concern. The letters had become increasingly more creepy. The words love and "watching you" had been used.
She had a decent idea of who it was and the principal contacted his parents and took him for some counselling. The letters stopped.
Two months go by and my cousin and the rest of her immediate family are coming home from a family dinner. Her older sister goes straight up to her room to change. She opens the closet and curled up in a ball in the corner is the stalker, crying and shaking.
No doubt my cousin flips the fuck out screaming bloody murder and my uncle rushes up. They knew who the kid was and although they were extremely mad they didn't get physical or anything. They called the police who came and took him away. They had a restraining order put on him and had him moved to another school. They didn't press charges because he was mentally unstable.
The creepiest part about the whole situation is that the whole family had been hearing noises in the house for months. The older brother of the family (whose room was in the basement) had heard the faint sound of doors opening and closing several times. And my cousin who he was stalking had noticed some of her personal belongings had gone missing, primarily underwear. In fact, it was an inside joke between the sisters for some time that she had to buy so many new pairs of underwear all of the time. The creep admitted that he had been breaking into their house for months to go through my cousins personal belongings.