OK, I have a little problem.

Well first of all dont put that letter in his backpack. Thats the worst possible thing that Du can do.

if this kid is leaving Du dont want to end things on a bad note even if Du dont get along well because Du might regret it a couple years down the road.

Du should try approaching him and asking why he hates you. Du either did something to make him angry oder he likes you. being mean to someone never solves the problem it can make things worse.
he might have a crush on Du and not know how to deal with it. talking is the best bet. if he Antworten angrily oder nastily then Du say something like "your loss" oder "you'll regret that attitude." he'll get your point.

Umadbro?

The Jedi way, that is not. Path to the dark side, that is. But who cares? this kids must go DOWN!!!!
Well...uh...hmmm. I don't know. Honestly I think Du could do way better than Schreiben a letter and not even giving it him face to face. I have a 'friend' like this. I'm assuming this is a dude that's giving Du crap and your a girl? He probably likes you. Anyway Weiter time he gives Du crap just say to his face, Okay you're either (feel free to add cuss words if Du feel that is nessisary) giving me crap because Du have a problem with me, I've done something to offend Du oder because Du like me. Then say in your meanest bossiest ball busting voice, WHICH ONE IS IT!? [Allow response] If it's that he has a problem with Du oder you've done somthing to offend him, calmly talk it out. If he likes you, explain that he's upset you, and Du want him to stop. If he says it's just a joke and to calm down, tell him in a an even Mehr ball-bustier voice than before, YOU'RE NOT FUNNY! STOP IT! Then if he doesn't stop all bets are off and Du are free to do what ever Du want to this dick. Don't wait untill the last Tag tho. I had a crush on this guy and told him on the last Tag before he moved to Finland, and I wish I hadn't even tho he didn't like me. You're going to want to see how he reacts. If he takes it well then you've made a friend. If not Du get to either watch him crawl away with his tail between his legs oder schlagen, punsch him the face. (Not that I encourage violence ;D)

tell him to Fu%k off