While students are unloading at a stop on the route, what must the driver do before pulling away to confirm everyone who should have gotten off actually did?

AAMVA CDL Manual, Section 10.2.3 – Unloading Procedures on the Route, p. 10-4 (July 2017 revision); 49 CFR §383.123(a)(1)

A miscounted or unaccounted-for student at an unloading stop is one of the most dangerous situations a school bus driver can face, since the bus may then move while a child is still near it. Actively counting students during unloading gives the driver verified proof, not just an assumption, that it's safe to close the door and leave.