Lunch periods
For the same reason that you’ll rarely have a whole-school gathering at an American school, you’ll not see the whole school eating lunch together in the cafeteria. There are just too many people to fit in all at once. This means we have lunch periods, different times when each year comes to eat.
For non-Americans, this can seem pretty weird, as they’re used to eating altogether and getting on with the rest of the day. I feel sorry for our lunch ladies who have a 2 hour-long serving time. At least they don’t have to deal with the whole school descending on them, though.