How to improve my jump and blocking in volleyball?

I'm a 15 yrs old boy, 5,7 and still growing...I'm Senior in high school and play volleyball for my school... I'm an all around player but my position in the team is a middle hitter (3). I became a middle hitter because of my quick plays and good timing in blocking.... my problem is i jump low, my reach are 8,4-8,6 in blocking and 8,8-8,9 in spiking... This coming months the try outs for college volleyball will start. What can i do to improve my jump and blocking in volleyball? Exercise?