The schools I work in, the social norm is to use Miss or Sir. Teachers also use these terms when in front of each other and pupils ( e.g. to a pupil - ‘did you check with Sir that you could leave the classroom’. To each other ‘Sorry to interrupt Miss, I need to speak to John Smith).
Intead of introducing Mx, why isn’t a universal title introduced that can be applied to all and all 3 be permissible. Pupils and staff can either use Miss or Sir in line with the staffs known sex, and the neutral one if they prefer or the staff specify. If a trans staff member prefers Miss, pupils should be allowed to use Miss or the neutral one.
Whilst a universal label could be used to denote a teacher / adult to replace