Ok I was just wondering what is the best way to build musle for hopping on a unicycle 
obviously lots of quadricep, hamstring, abdominal and tricep muscle exercises are good so get them powerful and you have good foundation,
Ryna Atkins world champion, is also one of the best wrestlers in high schools in canada powerful as anything, even almost me lol
Joe Hodges and xav both tuck huge infront so they are quite flexible and i asume must have quite good hamstring and abdominals to pull it up so high infront.
Plyometrics is the exercise technique used by bike trialists and i am trying it at the moment for my a level pe personal exercsise programme, look it up on google theres loads of tips.
Also dont want to sound patronising but bear in mind your only 13 so your body and muscles, and joints have not fully developed so sometimes best to see how you develop and just keep doing trials the higher hops will come with age and practice not just muscle its largely technique.
Hope that helped i feel really inteligent qwriting so much but its probably all bull lol it shouldn’t be
Yeh I’ll think i’ll just go on several 4km rides lol I’ll also do a bit of box jumping.
i just started unicycling yesterday and i can hop pretty good… mostly cuz im a pro pogo sticker…
so pogo sticking helps.
I find its also a good way to kill your knees! 
Well on my crappy one it is I assume u have a good one:D
Edit: funny how you can hop high but not ride a unicycle lol
anybody else got any ideas?
Just ride man. You are still young, Lucas is right, the more you ride, the better your techneque will get, and as you get older your muscles will develop naturally with your riding.
I try and ride for at least half an hour every day, and it is starting to show. My max hop height has greatly improved over the last few weeks.
Keep at it, you will improve and not even notice.
Rock on!
Edd
Thanks for all the advise from everyone!