At the school campus, when may a school bus driver leave the driver's compartment while students are loading?

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

Removing the key before stepping away closes off the one way an idle, driverless bus could unexpectedly move. This lets the driver actively supervise loading at a busy campus stop without creating a new hazard by leaving the bus unsecured.