OK, I have a little problem.

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

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

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

tell him to Fu%k off