We have a team of U13 girls and want to incorporate metrics in order to help them become better travel volleyball players. One of these metrics that we think is important is measuring their reach ability, I.e. Free standing, jumping, running jump, etc. Is there a proper method / or standard procedure for measuring reach in volleyball ?

Thanks for your help.

Get one of those stands that measures how far you can jump.
Something that works is have them develop stronger leg muscles. That will increase their jump. Also if you have punishments for them, like if they lose a drill during practice, have them do jumps at the net or something with strengthening their ankles.

Hope this helps