A volleyball player who needs to jump higher.?

I play 8th grade volleyball for my school and my coach put me as middle blocker. I'm really good at tips and blocks but most of the time I can't jump high enough to block the ball. I'm almost 5 foot 4... But still one of the shortest. I really don't want to play outside and if I can't jump high enough my coach might change to an outside player. How can I jump higher? Are there any exercises?