headmaster vs head teacher

headmaster and head teacher both are nouns.

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

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

Other hypernyms of headmaster include adult male, man, head, principal, school principal.

headmaster (noun)	head teacher (noun)
presiding officer of a school	the educator who has executive authority for a school