To monitor tail swing and avoid a tail-swing crash, when should a school bus driver check the mirrors?

AAMVA CDL Manual, Section 10.7.4 (Tail Swing), p. 10-11

A single glance in the mirror before starting a turn doesn't account for how far the rear of a long vehicle swings out mid-turn. Continuous mirror checks throughout the turn let the driver catch a pedestrian, cyclist, or object that moves into the swing path after the turn has already begun. This habit is the core defense against tail-swing collisions on a vehicle whose rear overhang the driver can't see directly.