See YouTube video - No-ball box-out, also Jr. NBA - Rebounding.
4 lines, the first person in each line (the defender) turns and faces the second player (the offensive player). On a box-out, defenders yell "shot", locate their check, step toward them, tag them with a forearm (don't extend or push out), turn (reverse pivot, be low), hold the box-out, go get the rebound with two hands, jump as high as they can, trying to get the ball at its highest point.
When coach yells "shot", defenders locate, tag, turn, hold the box-out for about a one-count, pursue the rim, yell "rebound", jump as high as they can with two hands, try to punch the backboard or touch the net.
(Variation - coach passes to a defender)
Rotate attackers to defenders, defenders to the back of the line.