“Highway” means a lot of things depending on the context. Sometimes it’s an interstate system with on and off ramps, but it also refers to a non-residential road (though in western New York, I’ve seen houses with driveways that connect with I-84, so who the fuck knows what the deal there is).

For the purposes of stopping for buses/emergency vehicles, a divided highway is any nonresidential road with a raised median such that a car is unable to turn around at any given place. At least in my state.

The only "highway" in the US where you're absolutely not allowed to stop (under normal circumstances) is a "controlled-access highway". The majority of which is the US Interstate system. Some state highways are controlled-access as well.

But I feel it's important to clarify: School buses do not stop to pick up children on those sort of roads. That would be insane.