schoolmistress vs adult female

schoolmistress and adult female both are nouns.

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

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

Other hypernyms of schoolmistress include school teacher, schoolteacher, woman.

schoolmistress (noun)	adult female (noun)
a woman schoolteacher (especially one regarded as strict)	an adult female person (as opposed to a man)