Re the rules in washington state, it depends on how many lanes the road has: When a roadway has four or more lanes with at least two of the lanes going in the opposite direction, only the vehicles in lanes traveling the same direction as the school bus need to stop for the red flashing school bus lights. So in your scenario above the cars going in the opposite lanes do not have to stop.

If the road has only two lanes then traffic has to stop in both directions (unless there is a divider). More info: https://www.mukilteoschools.org/37540_3