Why were the Meriweater kids all in the yard on their last day?

From the text:

Take my first class today: English. The whole middle school had read this book called Lord of the Flies, where all these kids get marooned on an island and go psycho. So for our final exam, our teachers sent us into the break yard to spend an hour with no adult supervision to see what would happen. What happened was a massive wedgie contest between the seventh and eighth graders, two pebble fights, and a full-tackle basketball game. The school bully, Matt Sloan, led most of those activities.