Some scholars services are very well used and provide links which otherwise wouldn't exist or would require a change of bus.

When I first started on the school runs, the 454 was my regular run (Easington Lane - St Roberts). I took in excess of £90 per day just on the school run but that was a full bus (90 kids).

St Roberts have the 458 which is (or at least was) tripled. 2 deckers used to leave with all the buses at school kicking out time then a 3rd bus (usually single deck) leaves shortly after. Again, thus journey wouldn't be available on a service bus (without changes) and also you wouldn't really want all those kids on a service bus.

Kids regularly vandalised the vehicles and acted in an antisocial manner - again what impression would a passenger on a service bus make of this?

Now I driver the 4274 Coxhoe to Belmont. There are no tickets on here and its a closed door service in the sense that it's only open to kids from the school with the appropriate pass (either bus A or B). This differs from the Nexus runs which were (may have changed due to Covid) open to the public and adult fares were available. Kids on the Belmont run are better behaved than St Roberts but 7157 has been internally vandalised so again would pitting these kids on a service bus leave a good impression on other passengers?

I think school buses have there place and most are quite well used (more than some works services I've driven - 968 springs to mind). When I went to school there was a school bus (didn't go where I lived) but it was taken off due to vandalism (running theme here) and we used service buses. Buses were packed and drove past other waiting passengers further along the route.