Sufficient seating space must be provided so far as practicable for each passenger transported inside each school bus, an aisle in the school bus must not be less than twelve inches in width and all seats must be securely fastened to the floor or body of the vehicle. All students must be within the body of the bus at all times while the bus is in motion. Students are not permitted any place outside the bus and may not ride with heads or arms protruding through open windows. The number of students assigned to a school bus must not be greater than the manufacturer certified seating capacity, and all passengers transported must have adequate seating area to comply with the occupant protection performance standards required in the Federal Motor Vehicle Safety Standards. Provided, however, that a limited number of excess passengers on regular routes may be permitted until the bus routes can be adjusted to accommodate the overload but not to exceed twenty school days.