school bus vs bus

school bus and bus both are nouns.

Word	Noun	Adjective	Verb	Adverb
school bus	Yes	No	No	No
bus	Yes	No	Yes	No

As nouns, bus is a hypernym of school bus; that is, bus is a word with a broader meaning than school bus:

Other hypernyms of school bus include autobus, charabanc, coach, double-decker, jitney, motorbus, motorcoach, omnibus, passenger vehicle.

school bus (noun)	bus (noun)
a bus used to transport children to or from school	a vehicle carrying many passengers; used for public transport
	a car that is old and unreliable
	an electrical conductor that makes a common connection between several circuits
	the topology of a network whose components are connected by a busbar

school bus (verb)	bus (verb)
	remove used dishes from the table in restaurants
	ride in a bus
	send or move around by bus