What is the best why to tell a guy "no"?

"I am going to tell the teacher that you are sexually harassing me. "

You can tell him that you appreciate the attention, but you aren't looking for a boyfriend right now, and that you'd like him to stop asking you out.
If that doesn't do it, then you can say it in a not-so-nice way.

If he keeps bugging you don't worry about hurting his feelings, he'll get over it. Sometimes when you're nice about things and beat around the bush, your point doesn't get across like you want it to. Flat out tell him no thank you, you're not interested, and leave it at that. If he keeps it up, report him. Guys that are that persistent sometimes need that kind of kick in the ass to tell them you're not into them.

Well if you have already told him no every way you can possibly think of. Then you might have to be more firm. Guys don't pick up on our subtle let downs. Lol you could always tell him your not into guys! ;)

If he's doing it every time he sees you then he's probably doing it just to be funny. I don't see any guy who would do that more than once especially if you give the sign that you're not interested. But yeah just tell him no, that you're not interested and tell him to stop asking.

Just say no and don't worry about hurting his feelings. That's just rude if a guy keeps asking you out and you've had to turn him down 50 million times. What does he think, he can wear you down or something? He'll live.

Say, look, I think you are a really nice guy, but I'm just not interested. Be up front and make sure you get your point across.