You are highlighting a serious problem in the UK, JD. People have not joined a Trade union or professional body. I find it worrying what they are asking for, even if to others it may seem bizarre. I'm concerned for your colleagues who are non UK citizens. I think some of the information requests you have highlighted, are borderline in breaching employment and E&D legislation. Hope it goes well for you all but I am a little worried by the behaviour of the Ttust.
My comments for her to join a union have fallen on deaf ears. She also won’t ask them why they are requesting this as she doesn’t like to rock the boat. Hopefully, her school are just being cautious and not malicious.