bus vs school bus

bus and school bus both are nouns.

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

As nouns, school bus is a hyponym of bus; that is, school bus is a word with a more specific, narrower meaning than bus:

Other hyponyms of bus include minibus, trackless trolley, trolley coach, trolleybus.

bus (noun)	school bus (noun)
a vehicle carrying many passengers; used for public transport	a bus used to transport children to or from school
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

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