What must be true before a school bus driver loading students at a route stop begins moving the bus?

49 CFR 383.123(a)(2); AAMVA CDL Manual, Section 10.2.2 'Loading Procedures,' p. 10-4

A bus that starts moving while a student is still standing or walking down the aisle risks that student falling, especially during braking or turning. Waiting for everyone to be seated and facing forward is a simple, verifiable check that prevents this specific injury.