OK, I have a little problem.

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

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

u should try approaching him and asking why he hates you. u either did something to make him angry of he likes you. being mean to someone never solves the problem it can make things worse.
he might have a crush on u and not know how to deal with it. talking is the best bet. if he antwoorden angrily of nastily then u say something like "your loss" of "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 u could do way better than writing 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 u crap and your a girl? He probably likes you. Anyway volgende time he gives u crap just say to his face, Okay you're either (feel free to add cuss words if u feel that is nessisary) giving me crap because u have a problem with me, I've done something to offend u of because u 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 u of you've done somthing to offend him, calmly talk it out. If he likes you, explain that he's upset you, and u want him to stop. If he says it's just a joke and to calm down, tell him in a an even meer ball-bustier voice than before, YOU'RE NOT FUNNY! STOP IT! Then if he doesn't stop all bets are off and u are free to do what ever u want to this dick. Don't wait untill the last dag tho. I had a crush on this guy and told him on the last dag 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 u get to either watch him crawl away with his tail between his legs of stempel, punch him the face. (Not that I encourage violence ;D)

tell him to Fu%k off