First off be careful. Do you know football players (jocks), tough guys at your school? Tell them and it would work much better than you doing it. Teachers never want to get involved.
I had a talk with a guy doing this to one of my ex girl friends. I was coaching a soccer game and I saw her and her kids there (we were broken up for many years). Anyhow she was crying and said that his guy keeps following her everywhere and she was afraid for her kids and herself.
I was hot mad and ready to knock this guy out, BTW don't tell someone who won't do shit, they will need to be prepared to beat this guys ass and should be ready for any consequences involved (i.e. going to jail or the guy doing something). While I'm 40 I will gladly whoop someones ass if they're fucking with family or friends or doing unjust shit like your situation. I'm 6'2 and I'm undefeated in fights since the 3 grade :)
So jump a fence and walk up to his truck and said, so why are you following her? He said who? I'm not following anyone. (She told me he was married and lied to her) I said look homie you're married and you need to back the fuck off. She has kids and she's getting very worried that you're going to harm her or them, what the fuck are you doing?
He started to make an excuse so I cut him off and I asked him does he have kids? He said yes a daughter. I was like homie what would you do if someone was stalking your daughter? I said please stop this bullshit man, I told him I try and stay non-violent but you can't keep this shit going on, you need to make up with your wife or get a divorce but the point is to move on.
I told him he's acting like a bitch, I said you're a fucking man acting like you're in grade school, stop fucking around before I have to beat your ass. Do you understand that you need to leave her the fuck alone?
He could see that I was going to punch him any second.
He said he would and I told him now please leave and I don't want to have to hear about this shit again because I know way too many people in town and so do you so let's just move the fuck on with life so this story doesn't have to be in the papers.
He drove away and never bothered her again.
I have a daughter and I would just suggest that you put this into the hands of a classmate with balls. I would never tell her to handle this on her own. If there aren't classmates around that can actually defend you then unfortunately you're going to have to get a friend to record his behavior then give is to the police.
What if they don't do anything? PM me and I'll get it handled, I'll talk to your principal and all types of people.
Hope that helps, be safe!