OK, I have a little problem.

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

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

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

tell him to Fu%k off