What does the round, black-on-yellow advance warning sign placed ahead of a railroad-highway crossing tell a school bus driver to do?

AAMVA CDL Manual, Section 10.4.2 – Warning Signs and Devices (Advance Warning Signs), p. 10-7

The advance warning sign gives drivers lead time, well before the tracks, to begin slowing and scanning, which is critical for a vehicle as long and slow-stopping as a school bus.