If a school bus driver must leave the driver's seat to address a serious behavior problem, what must they do?

AAMVA CDL Manual, Section 10.5.2 (Handling Serious Problems), p. 10-8; 49 CFR Part 380, Appendix D, Unit D1.7 (Student Management)

An unattended bus with the key still in the ignition is a safety and liability risk, especially with children aboard. Taking the key when stepping away from the seat prevents the vehicle from being moved by anyone else while the driver is dealing with the behavior problem.