schoolgirl vs adult female

schoolgirl and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
schoolgirl	Yes	No	No	No
adult female	Yes	No	No	No

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

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

schoolgirl (noun)	adult female (noun)
a girl attending school	an adult female person (as opposed to a man)