This is a drill that always helps me.
Get one person to shoot the basketball, have two or more in a rebounding position. When the shooter shoots the ball, see who can get the rebound.
It can work at any age, just try to do it around 10 times every practice.
I never make the kid run who gets the most rebounds, it always makes everyone try harder.
You would need 3 or more

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TLC: Pål Degerstrøm 2000-2008