In each serious accident involving a school bus or a student transportation vehicle as defined in section 14-212, the police officer who, in the regular course of duty, investigates such accident, shall immediately report such accident by telephone or otherwise to the Commissioner of Motor Vehicles. In the event of any accident in which an occupant of a school bus or student transportation vehicle is injured resulting in admission of such occupant to a hospital overnight, the police officer investigating the accident shall report such accident to the commissioner within twenty-four hours thereafter. For the purposes of this section, the term “serious accident” means any accident in which (1) any occupant of the school bus or student transportation vehicle is killed, or (2) a fire occurs in, or there is a roll-over of, the school bus or student transportation vehicle.