There is a line between being persistent and being a pest, and it sounds like this guy has completely crossed it. He clearly isn't getting the hint that you're not into seeing him every time you turn around — he probably thinks that if he's constantly by your side, you'll eventually realize how wonderful he is. He needs to know that's not the case. The next time he sidles up to you while you're talking to another guy, take him aside and tell him exactly how you feel. You can be blunt ("You're making me uncomfortable!") without being mean ("You're totally freaking me out!"). Chances are, he's just trying way too hard and doesn't realize he's skeeving you out. Once you tell him that his attempts to win you over are having the opposite effect, he'll probably stop.

However, if he's still lurking around your locker after you've told him how you feel, you may have to get other people involved. Not to scare you, but any time a guy goes from stalker-ish to stalker, you have to be concerned. Since you have mutual friends, you can't just avoid him forever, and you shouldn't have to. Ask someone you trust, like your school counselor, to speak with him about why he needs to back off. And if this guy seems even slightly scary, or if he threatens you in any way, trust your instincts and go straight to a teacher, your principal or your parents, and let them know what's up. They can help you make sure he doesn't bother you anymore.