Emergency vehicles have devices that announce their presence to get traffic lights to change in their favor. “Kids being late to class” is not on the order of importance to create a complex scheme to change traffic lights based on strobe lights from a bus.
Bus priority lanes and traffic lights that give priority to busses are definitely a thing. Usually for municipal busses and not school busses, but I'd expect a community that had priority lights for busses would allow school busses onto the system as well.
Not specifically to avoid late arrivals of pupils, but because prioritizing many passenger vehicles is valuable.
Sounds like urban legend.