OK, I have a little problem.

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

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

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

tell him to Fu%k off