When the flashing red lights mounted on a cross-buck sign begin to flash as a school bus approaches a railroad crossing, what must the driver do?

AAMVA CDL Manual, Section 10.4.2 (Flashing Red Light Signals), p. 10-8

Flashing lights are the earliest active warning a driver gets at a powered crossing, often before a train is visible or a gate has finished lowering, so treating the lights alone as a mandatory stop signal is what keeps a slow-stopping school bus off the tracks in time.