headmistress vs head teacher

headmistress and head teacher both are nouns.

Word	Noun	Adjective	Verb	Adverb
headmistress	Yes	No	No	No
head teacher	Yes	No	No	No

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

Other hypernyms of headmistress include head, principal, school principal, adult female, woman.

headmistress (noun)	head teacher (noun)
a woman headmaster	the educator who has executive authority for a school