Bus operators will often find it economical to convert retired buses to use as permanent training buses for driver training, rather than taking a regular service bus out of use. Some large operators also converted retired buses into tow bus vehicles, to act as tow trucks.
Explain to an 8 year old child that is not yet 49 tall that the safety equipment built into the vehicle is for adults  not children, and a child needs to continue to use the proper equipment (booster seat) so that the adult equipment will fit, and be able to protect, their bodies in the event of a crash.
Six seats had the more reasonable minimum weight limit of 40 pounds. This makes sense because the growth of children between 30 and 40 pounds occurs at a slower rate than when they are infants. An average 40-pound child is typically closer to age five, a much safer age to consider making a transition to booster use.