This isn't universal. The schools in our Montana town have pickup lanes and short term parking areas for pickup. Stopping on the road isn't allowed.

For a school near me, the road is no parking during pick up/drop off times. It even changes to one way traffic. The no parking windows is similar to alternate street sweeping days. There are signs posted that indicate the times.

Same for my tiny town. Stopping on the road is 100% not allowed, and parking isn't allowed there either. The school has its own parking area to park and pick up/drop off kids, and cars in there creep at 2 or 3 MPH.