girl vs schoolgirl

girl and schoolgirl both are nouns.

Word	Noun	Adjective	Verb	Adverb
girl	Yes	No	No	No
schoolgirl	Yes	No	No	No

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

Other hyponyms of girl include Campfire Girl, farm girl, flower girl, moppet, Scout, flower boy.

girl (noun)	schoolgirl (noun)
a young female	a girl attending school
a girl or young woman with whom a man is romantically involved
a youthful female person
a friendly informal reference to a grown woman
a female human offspring