What do the amber (yellow) warning lights and then the red flashing lights on a school bus signal?

The amber (yellow) flashing lights are activated in advance to warn other drivers that the bus is about to stop to load or unload students; they are a signal to prepare to stop. When the bus actually stops for students, the driver activates the red flashing lights and extends the stop arm, which legally requires traffic in both directions (subject to specific divided-highway rules) to stop and remain stopped until the lights are off and the stop arm is retracted. The driver should activate the amber lights far enough ahead to give following traffic time to slow and stop, then switch to red at the stop.