schoolgirl vs girl

schoolgirl and girl both are nouns.

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

As nouns, girl is a hypernym of schoolgirl; that is, girl is a word with a broader meaning than schoolgirl:

Other hypernyms of schoolgirl include female child, little girl, adult female, woman.

schoolgirl (noun)	girl (noun)
a girl attending school	a young female
	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