Comment by bonoboTP

I don't think 6-8-year-olds would cause a lot of mayhem. But parents need to go to work and kids aren't trusted to supervise themselves at that age (analogous to kindergarten before it). For teenagers the mayhem thing is also doubtful because school usually ends around 1-2 pm, while parents only finish working around 5 pm, so there's plenty of time to roam in the afternoon (at least outside present day America where parents drive kids around until late teenage years - which really is rather the exception in the broader context of school tradition).