OK, I have a little problem.

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

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

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

tell him to Fu%k off