120 at 5'8? That's good for jumping ability I guess Work on your legs by doing calf raises and jumping drills. Don't use weights because it will stunt your growth. That's what happened to me. Keep practicing and you'll be dunking in no time
Lifting weights doesn't stunt your growth . That is a myth .!
But , at that height, you should not worry about your jumping ability. You should train to perfect your shot, and ball handling . Work on your guard skills, and get quicker. You'll be hard to stop because you're so small. Use it to your advantage, and not an excuse .!
Just think when you DO GROW, you'll have amazing guard skills. Just think about Lamar Odom or Kevin Durant. Set your standards high, and put in the work .!
One way however that is probably the easiest to jump higher is to just jump .! Work on your form and technique, because you will jump higher. Practice jumping, and using a jump rope will help you on your speed and quickness and your jumping ability at the same time and your endurance .!
Hope I helped .!
A great exercise you can do for vertical is speed squats. You can either use just an empty bar or put very low amounts of weight on it. All you do is just do as many squats as you can for 1 minute. Make sure you go low like a normal squat, except you do it as fast as you can.
Your fellow lover of the game,
Hard work beats talent when talent doesn't work hard.
- Kevin Durant