As nouns the difference between headmistress and heiress
is that headmistress is a female school principal while heiress is a woman who stands to inherit.
headmistress
English
Noun
(es)
(chiefly, British) A female school principal.
Synonyms
* headteacher (UK, gender-neutral)
* principal (US, gender-neutral)
heiress
English
Noun
(es)
a woman who stands to inherit