In my PE classes, most problems start by somebody "forgetting" the rules. Generally speaking, kids are forgetful and get too into the activities and forget. Reminding them the rules helps them. Sometimes they're breaking the rules to gain an advantage. Some might call it cheating, but that's a forbidden word in my gym. Kids are smart, and wily. They will test the limits and see what they can get away with. They'll see if anybody notices. Then they'll see if anybody has the stones to do anything about it. They'll find loopholes and ways to work the rules into their favor. Sounds like just about every successful person in the history of the world, if you ask me. If they're using the rules to their advantage, that's great! Isn't that what we want our kids to do? If they're intentionally breaking them to gain an advantage, that's what we don't want. But if you call them out on it, or call them names (such as "cheater"), they get defensive. Then, they either act like they didn't do it, say they forgot (or didn't know in the first place), or bring to light another problem they see somebody else doing. But, instead, if you "assume the best" out of them, you'll typically get the best out of them.