What would be a gender neutral form of address as a highschool teacher? (British English)

In part of your reply, you describe a couple of titles that would be inappropriate for teachers to use: doctor or professor. While that is true, there are other titles that may be appropriate at the high school level: teacher or teach.

Teacher is sometimes used as a proper noun in American English, so I decided to try to find it in British English. The British National Corpus gives one example of "Teacher" as a proper noun, from what appears to be the start of a letter:

Dear Teacher I am glad to have the opportunity of welcoming you to WISE V ...

Of course English rock band Pink Floyd was doing this decades ago in "Another Brick in the Wall":

Hey, teacher, leave them kids alone

I've also seen this abbreviated to Teach, as this example from (American) TV show The Good Place, Season 3, episode 3:

Hey, teach, I've been having so much fun here, but I didn't plan to stay in Australia this long, and I'm super broke.

Teach feels informal and colloquial; teacher more formal.

It's not widely known, but it is recognised by the UK Government:

https://en.wikipedia.org/wiki/Mx_(title)

(pronounced Məks) eg Mx Smith. It could presumably be used in the form "Please Mx, can I have some more?"

Edit: I have never head this title actually used, but it appears to be the title that could be used if a person insisted on a gender-neutral title.