Per the AAMVA manual, when should a school bus's roof-mounted white strobe lights be used, if the bus is equipped with them?

AAMVA CDL Manual, Section 10.7.1 (Strobe Lights), p. 10-9-10-10

Strobe lights are a visibility tool, not a general-purpose warning light, so using them outside of actual limited-visibility conditions can run afoul of state or local rules governing their use. Knowing the specific trigger, limited visibility around the bus, keeps their use both effective and compliant.