Welcome to fall in Japan. A season for school undokai, that is, athletic competitions.
When I was invited to the undokai for the child of a church member, I anticipated something of a smallish scale. My experience with undokais had been 50~60 people or so gathered in a park to watch and cheer on their kids as they run relays, jump hoops, pull tug-a-war ropes, and so on. Imagine my surprise when the undokai I had been invited to involved 3000 adults and some 300 kids.
With this many involved, it's pretty hard to include everyone without injury. Yet the kids marched in formation, danced in rhythm, and did various group-oriented athletic activities that could only be possible in group-oriented Asia. The day was a great cultural study. Take a look at this short video of the kids dancing in formation to the music. You have to see it to understand.