Toughest problem I ran into when coaching middle school was getting them to rebound. At that age it's usually just the biggest kids get the boards. Boxing out is something younger kids can learn the technique of but forget all about it during a game. Like they never learned it.
I ran a drill (that my high school team ran) where I'd take 4 players on offense with me and play half court, with me shooting a perimeter shot and the defense had to box out and rebound. If the offense got the board, defense did push ups.
It improved the aggressiveness for rebounds but I had kids colliding and getting hurt. Turned into a mess.