The phrase "assistant master" is correct and usable in written English.
It can be used in educational contexts to refer to a teacher or educator who assists a headmaster or principal in managing a school or educational institution.
Example: "The assistant master was responsible for overseeing the students during lunch and ensuring they followed the school's rules."
Alternatives: "deputy headmaster" or "associate teacher".
"It's a real trashy spot right here," said Randy Jowers, assistant master on the vessel.
Upon finishing his studies in 1947 at St Mary's college in St Lucia, he immediately became an assistant master there.
He worked as an assistant master at his alma mater, St. Mary's College, teaching Roman and British history, and worked on his writing.
Mr. Taylor, 24, is an electrician who works on theater productions, most recently as the assistant master electrician for the Public Theater production of "The Book of Grace".
Borden cut short his formal education before his 15th year, when he accepted the post of assistant master of the private school he was attending.
On graduating in 1955 he played for London Scottish's 1st XV and married Daphne Simpson before joining the staff at Harrow as an assistant master to teach history.
I never saw the revue itself (though I have a recording of it on CD), but I did see the play, starring Paul Eddington as the headmaster of Albion House and Bennett himself as one of the assistant masters.
Control was originally vested in six parishioners of standing, but under the Public Schools Act of 1868 the governing body consisted of six members elected respectively by the lord chancellor, the universities of Oxford, Cambridge, and London, the Royal Society, and the assistant masters of the school.
My brother Michael, five years older than I, had been going to a day school near our house in West Hampstead, and when it was closed at the outbreak of the war one of the assistant masters there decided to reconstitute the school, in a little village I will call Greystone.
Self-educated, he became assistant mathematics master at the Royal Military Academy, Woolwich, in 1801.
After 1949, when he became its assistant chorus master, the Metropolitan Opera was his main employer.
