Here's what happens:
Gym teacher absent. They don't expect a substitute to do exercises, dodgeball, etc.
You do what's called "In-class" phys ed. The regular teacher prints 300 copies of a word puzzle and gives you a Nerf ball. And you make housecalls to different classrooms instead of meeting in the gym.
Now you have to "sell" the program to kids from grades 1 to 8. Nobody wants to do it. What I do: I take a survey of how many kids are involved in sports. About half, usually the boys, raise their hands. I go into the shpiel about what it takes to win at sports. Aptitude, performance, and . . . . strategy. You have to be smarter than the other team. They're buying it. I continue that these puzzles are practice for strategy, seeing things that are not obvious to the eye.
OK, sometimes this works, sometimes not so much.
And that takes about 20-25 minutes of a 50-minute period. Then comes Silent Ball.
The kids get on their desk tops and throw a Nerf Ball to each other. And of course it's never "silent".