I would start by ensuring the players have the right body form for a good take off (do a search on the net to get some good tips on how a good take off looks) but the basics are, soft knees, arms bent beside body and ready to be used in the launch up, eyes up and looking at the ball.

you can do some basic drills to work the calf muscles so they are trained on getting good lift.  a really good start is to find a nice high brick wall or wire fence and have the players face side on to the fence/wall.  using the technique you have found on the net for take offs, have them lift up as high as they can go 10x and touch the wall with both hands extended up but only one touching the wall (the one closest). tip...If you put chalk on their fingers you can mark where they touch.

Mark the wall for each one on day one, then after practising this (do this both sides) for a month see if they have improved.  (you can buy an elevation tree marker that they jump in front of and push the tree limbs at the highest point.

can also do some high pass catching drills from a stationary position, you can have them throw the ball above their head and catch it as high a point as they can.  have them rebound every shot no matter if it goes threw the goal ring or not.

hope that helps.