How can I jump higher? Please help?

I'm a 14 year old guy, 6'1, and 155 lbs. I have a pretty low vertical (only about 24 inches). I have used multiple exercises, and yet i can only barely touch the rim. There are guys shorter than me already dunking! And everytime at bball practice we do layup drills and my coach always yells "DUNK IT!" and one day at practice i want to just throw it down. Please help! I think my biggest problem is that I dont stay commited.