Federal regulation does not specify the number of persons that can sit on a school bus seat. The school bus manufacturers determine the maximum seating capacity of a school bus. The manufacturers use this number, which is based on sitting three small elementary school age persons per typical 39 inch school bus seat, in the calculations for determining the gross vehicle weight rating and the number of emergency exits. School transportation providers generally determine the number of persons that they can safely fit into a school bus seat. Generally they fit three smaller elementary school age persons or two adult high school age persons into a typical 39 inch school bus seat.

NHTSA recommends that all passengers be seated entirely within the confines of the school bus seats while the bus is in motion. Federal motor vehicle safety standard No. 222, "School Bus Passenger Seating and Crash Protection" requires that the interior of large buses provide occupant protection so that children are protected without the need to buckle-up. Occupant crash protection is provided by a protective envelope consisting of strong, closely-spaced seats that have energy-absorbing seat backs. Persons not sitting or sitting partially outside of the school bus seats will not be afforded the occupant protection provided by the school bus seats.

Source: NHTSA