Besides ensuring students are far enough from the bus that it cannot topple on them, what other hazard should a driver avoid when choosing a tornado-evacuation shelter site?

AAMVA CDL Manual, Section 10.3.2 (Evacuation Procedures), p. 10-7

Ditches and culverts, the very features recommended for tornado shelter, are also where flash floodwater collects fastest during severe storms. A driver must weigh both hazards at once rather than treating tornado shelter-seeking as automatically safe.