If a white stop line is painted on the pavement before a railroad crossing, where must the front of the school bus remain while stopped?

AAMVA CDL Manual, Section 10.4.2 (Pavement Markings / White Stop Line), p. 10-8

Because a school bus is long and cannot stop quickly, keeping its front end safely behind the painted stop line ensures the entire vehicle, and all its passengers, stay clear of the tracks even if the driver misjudges distance.