The overhead inside rearview mirror on a school bus primarily helps the driver monitor which of the following?

AAMVA CDL Manual (nhcdm.pdf), Section 10.1.6 – Overhead Inside Rearview Mirror, p. 10-3

Because the overhead mirror is the driver's only view of what's happening behind them and inside the bus, regularly checking it during loading, unloading, and while driving is a key habit for catching unsafe passenger behavior or a student still in the danger zone.