I'm presuming this is the first teaching job they've had? If not, what did the pupils at their previous establishment call them? (Directly to their face, I mean.)
If this had been a thing when I was a kid I'd have lived in daily fear of getting it wrong and probably just kept my mouth shut and not participated at all in that person's lessons though I'd definitely have had plenty of private opinions and it would almost certainly have affected how I felt about, and how well I did, at their subject.
I think the suggestions to contact the two organisations mentioned previously for guidance are very sensible.
I'm also curious about what subjects they teach - I'm guessing it's not something like some modern languages (French, say, where it matters if words are classed as masculine or feminine at least when you're writing the language) or a minefield subject like biology.
And I also wonder how much learning those poor kids are experiencing, call me old fashioned (in this respect. ⁰I work for an organisation where we bend over backwards to be accommodating and I'd put myself somewhere in the plus of LGBT etc, but I'm not at all precious about what I'm addressed as, as long as it's not Hoi You...)
Would suggesting this person be addressed as Professor, as if they were employed at Hogwarts, be neutral enough for them or is that too Potterphile? That's about as neutral a term as I can think of in English apart from something like Teacher or Teach. In Japanese the term sensei is used to address all teachers and I'm sure there are other languages where there are equivalents, I just can't think of them off the top of my head.
Or use first names, in which case wouldn't all teachers have to then be addressed by their first names, or as their full name, eg Jenny Jones or Alex Green or whatever.
And if this is causing them such consternation I would privately wonder if this teaching environment is the best match for them. Though they may be a brilliant teacher and if this is an exam subject they'll have the end of year results as an illustration.