Have everyone gather around the circle at half court, or use the circle at the free throw line.
Put a ball at the center of the circle.
Pair everyone up. In each pair, one is on offense, and starts on the outside of the circle; the other is on defense, and starts on the inside.
As soon as you blow the whistle, everyone on defense has to prevent his/her opponent from getting to the ball by continuously boxing out. Drill ends when someone gets to the ball in the middle.
Can work with any number of pairs, but if you've got more than ten kids, you might want to break it up into smaller groups.