At a railroad crossing equipped with automatic gates, when is a school bus driver required to stop?

AAMVA CDL Manual, Section 10.4.2 (Gates), p. 10-8; 49 CFR 383.123(a)(2)(iii) (railroad-crossing procedures as a required knowledge-test topic — general topic only); 49 CFR Part 380, Appendix D, Unit D1.6 (Railroad-Highway Grade Crossings)

School buses carry children who cannot protect themselves, so the margin for error at a rail crossing has to be built in before the danger is visible. Stopping the instant lights begin flashing, rather than waiting for the gate or the train itself, gives the driver room to react if the bus is moving too fast to stop safely at the usual point.