omnibus vs school bus

omnibus and school bus both are nouns.

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

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

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

omnibus (noun)	school bus (noun)
a vehicle carrying many passengers; used for public transport	a bus used to transport children to or from school
an anthology of articles on a related subject or an anthology of the works of a single author

omnibus (adjective)	school bus (adjective)
providing for many things at once