What must a school bus driver do with the transmission and parking brake at every single stop made to load or unload students?

AAMVA CDL Manual, Section 10.2.1 'When stopping,' p. 10-3; 49 CFR 383.123(a)(2)(i) (general topic only); 49 CFR Part 380, Appendix D, Unit D1.2 (Loading and Unloading)

A school bus with children walking directly in front of it cannot be allowed to creep forward even slightly. Securing the transmission and parking brake at every stop removes the risk of the bus rolling while the driver's attention is on watching students board or exit.