This is a great idea as I'm aware lots of students are being reminded/spoken to a lot about misgendering and it is a genuine mistake for them, they forget as my colleagues presents traditionally female.
To come at this from another angle, I think you are - quite unintentionally - participating in the wholesale gaslighting (which is a form of emotional abuse) of these students.
This is harmful to them. To be constantly pulled up for something they are pretty much inevitably going to do. To be told that they cannot trust the evidence of their senses, their own interpretation of the world; they are wrong for not censoring and pre-editing every single one of their interactions with this person, and for reading this person who presents as obviously female, as obviously female.
It seems to me this colleague is far more concerned with “their” own well being than that of the students. I would seriously question this person’s suitability as a teacher at all.
I realise that’s no help to you in dealing with the current situation but I just thought you might find it interesting to have another perspective. The adult is effectively claiming to be vulnerable relative to the students here, but that’s turning everything on its head. In the adult teacher/student dynamic, it is the young people who are more vulnerable - which safeguarding acknowledges.
What I see here is a teacher making the students prioritise “their” (singular) well being ahead of their (plural) own, and that’s not good. It’s a form of control.
How can we teach young people to spot the red flags of abusive behaviours in their personal relationships if we are allowing teachers and other staff in schools to behave in the very same controlling and gaslighting ways towards them with impunity? Not just allowing but enabling and encouraging?
I suspect you are used to living and working within a culture where it’s heresy to even ask any of these questions though, so I guess it won’t be easy to take this on board. And as I said originally, I am quite sure that you are acting from the very best of intentions.
It is worth asking yourself though if you feel your first duty is to the students in your care, or your colleagues?