Let's get a few things straight.
Speed cameras are placed in schools zones, but don't actually follow the schools schedule. For example, they are on until 11 pm every night. The school zone placement is more of a political than practical consideration. Tickets are also only issued starting at 36 mph.
Additionally, school zone speed limits are 20 only during lineup and dismissal times. And not every camera zone has that speed limit. A school zone for this purpose is about physical proximity, which is why a camera might be installed two blocks behind a school, though a 20 mph speed limit in that spot in useless.