in a girls' boarding school, I would probably expect mistresses to use Miss Surname.

According to the old school stories I used to read, many of which were pre-war, the girls were always first name only, to the teachers as well. It was so at the Marlow girls' school, Kingscote, which was post-war, and at by boarding school for that matter. What did they do at yours?

I would be angry too. Being addressed as Miss or Mrs also annoys me. It should be Ms, making it the same as Mr: merely an honorific and not an indication of one's relationship to a man.

I thought much the same about Hermione; she'd appreciate the indication of equality. :-)