Daycare or not, why is income relevant here? Rich or poor alike, daycare or learning alike, kids go to school for a multitude of reasons including socialization, applied skills (shoe-tying, sharing, etc.), and exposure to things besides television and iPads, none of which hinge on income.

Sorry, I should have clarified. Lower income people could not afford the cost of sending the children to daycare if schools actually sent students home after 3 hours each day.

Where I grew up, school became the social safety net for children from low income backgrounds. If the temperature dropped to -10F, they would still not cancel school because the low income kids received better heat and better meals from the school.

My point is that in many cases, school isn't nearly as much about learning as it is to have a safe and stable place for children from unstable households.

Why just "lower income". Why do you think pretty much every working couple of parents sends their kids to school with afternoon activities until 5 PM. AFAIK this is extremely common in the bay area.