Sufficient seating space shall be provided so far as practicable for each passenger transported inside each school bus, no aisle in the school bus shall be less than twelve inches in width and all seats shall be securely fastened to the floor or body of the vehicle. All pupils must be within the body of the bus at all times while the bus is in motion. They shall not be permitted to ride on the running boards or in any other place outside the bus, nor shall they be permitted to ride with heads or arms protruding through open windows. Commencing with the 1974-75 school year, the number of pupils transported on a school bus shall not exceed by more than ten percent the manufacturer's rated seating capacity of such bus; and by the school year 1975-76, seating space shall be provided for each pupil transported. Provided, however, that a limited number of standees may be permitted until the bus routes can be adjusted to accommodate the overload but not to exceed twenty school days.