
Quoting
iowa Code, Sec. 321.372(3)
The driver of a vehicle, including the driver of a vehicle operating on a private road or driveway, when meeting a school bus with flashing amber warning lamps shall reduce the vehicle's speed to not more than twenty miles per hour, and shall bring the vehicle to a complete stop when the school bus stops and the stop signal arm is extended. The vehicle shall remain stopped until the stop signal arm is retracted after which time the driver may proceed with due caution.
The driver of a vehicle, including the driver of a vehicle operating on a private road or driveway, overtaking a school bus shall not pass a school bus when red or amber warning signal lights are flashing. The driver shall bring the vehicle to a complete stop no closer than fifteen feet from the school bus when it is stopped and the stop arm is extended, and the vehicle shall remain stopped until the stop arm is retracted and the school bus resumes motion.