mistress vs school teacher

mistress and school teacher both are nouns.

Word	Noun	Adjective	Verb	Adverb
mistress	Yes	No	No	No
school teacher	Yes	No	No	No

As nouns, school teacher is a hypernym of mistress; that is, school teacher is a word with a broader meaning than mistress:

Other hypernyms of mistress include schoolteacher, adult female, woman.

mistress (noun)	school teacher (noun)
a woman master who directs the work of others	a teacher in a school below the college level
an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
a woman schoolteacher (especially one regarded as strict)