Consider two situations: moving the students could worsen their injuries, or hazards such as downed power lines are present outside the bus. In which situation is it unwise to evacuate?
Either condition alone is a reason to keep students aboard: moving a student with a possible neck, back or bone injury can make it worse, and sending students out near downed power lines or similar hazards exposes them to new danger. Evacuation is required in cases such as fire or a stall on railroad tracks, not in every emergency.