schoolgirl vs little girl

schoolgirl and little girl both are nouns.

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

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

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

schoolgirl (noun)	little girl (noun)
a girl attending school	a youthful female person