|| You can drive up to 16 passengers on a class "D1 category || restriction 1" which is what everyone who passed before 1/1/97 has, || PROVIDED it's not for hire and reward. Hire and reward has quite a || broad definition, though, but I doubt it includes volunteers driving || the school bus - unless you get paid for the said driving.
The "hire and reward" isn't for the driver, it's for the bus itself AIUI. If the passengers pay for the trip, then it's H&R, regardless of whether the driver is paid or not.
I used to drive a 16-seater Transit minibus and a 53-seater coach for my school on a car licence in the 70s/80s. The dodge was that pupils did not pay for the trips - instead they made a "voluntary" contribution to school fund, which then paid for the diesel. I wasn't paid, except in my capacity as a teacher in the school. That was well dodgy even then - I imagine that loophole (if such it was) has been closed by now.
BTW, I *loved* driving the coach.