We played "Buttball" a lot from like 1st to 6th grade. I don't remember the exact details but this is what I remember:
You'd need a tennis ball, a wall and at least 3-4 people... When the game got really popular at school there would be games of like 15-20 people. If you have too few or too many it doesn't really work, the number of people depends on the length of your wall.
The first person takes the ball, throws it against the wall, and everyone tries to catch it. If someone catches it in the air, the catcher throws the ball at the wall as the thrower runs to the wall. If the ball hits the wall first, the original thrower has to stand still, face the wall and have the ball thrown at them by the person that caught it. If the thrower gets to the wall before the ball, they get to throw again.
If the ball bounces before someone catches it, the original thrower is safe, and the person that caught it starts the next round by throwing the ball at the wall again.
If the person that's trying to catch the ball drops it, they have to stand against the wall and the original person throws the ball at the person that dropped it.
I heard that a couple years after I left that school they banned Buttball along with Smear the Queer, Bloody Knuckles and pencil fights.