OK, I have a little problem.

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

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

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

tell him to Fu%k off