What does the cross-buck sign at a railroad-highway crossing require a school bus driver to do?

AAMVA CDL Manual, Section 10.4.2 (Cross-buck Signs), p. 10-8

The cross-buck functions as a permanent yield sign for every crossing, active or passive, which is why drivers are trained to treat it with the same seriousness every single time, not just when a train happens to be in sight.