A seat assembly for a vehicle, such as a school bus, accommodates two or three passengers, depending on the size and stature of the passengers. The seat assembly is equipped with three seatbelts and five buckles spaced along a seatback. When three passengers are utilizing the seat assembly, each of the three seatbelts and three of the buckles are utilized. When two passengers are utilizing the seat assembly, two of the seatbelts and the remaining two buckles are utilized. The seatbelts and buckles are visually coded and/or keyed to prevent improper use. When the buckles are not being utilized, they are housed in cavities formed in the seatback to avoid interference with the passengers.