we have a bunch of buses running around burning more fuel and polluting more

An even worse factor is that EPA regulations governing diesel engine emissions specify allowed emissions in terms of horsepower-miles, which means that more powerful engines are allowed to emit more pollutants per mile. This has led to school buses becoming over-engined compared to what they used to be a few decades ago. This is good for lead-footed bus drivers and the people behind the buses (I guess), but not so good for the air. And of course EPA diesel regulations specify nothing about greenhouse gas emissions.

At least school buses have the intrinsic efficiencies of mass-transportation behind them. If parents were driving all these kids to school instead, the net emissions would be vastly worse. Of course most of these kids could walk or bicycle instead …