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.
I think switching to a high back booster around 4 years old for most kids is reasonable. Thats when we switched my 6 1/2 yr. old and 4 1/2 year old. My kids barely met the weight/height requirement at that age and were trustworthy enough to not try to unbuckle etc.