I would "reasonably" expect children to be present in a school zone an hour before school starts and maybe a couple of hours after school ended.
I wouldn't consider it reasonable to expect children to be present while school was in session -- I mean, they're inside being taught, right?
Exactly my complaint with these zones.
If a playground area is directly adjacent to a road, then there is an argument for enforcing the lower limit through the whole school day on that road.
Otherwise the limit should only be in place during the start and end of the school day.