The statutory KCSIE (Keeping Children Safe in Education) guidance is a good place to start. Paragraphs 205-209 link to the Cass Report, which adds a greater level of detail on why the statutory schools' guidance describes gender questioning children as vulnerable.
The current statutory guidance also then links to the draft Gender Questioning Children guidance. This is where the information about best practice on residential school trips is held. Unfortunately, this specific document is still in draft but in the absence of any other best practice on residential trips, it's a good place to start.
Personally, my approach would be to write to both the school and the council and:
- Make it clear that you are referring to current statutory guidance in the first instance (KSCIE)
- Say that although the GQC guidance that is referenced within it isn't yet finalised, it is the only current guidance that is currently available...
- And then ask them to confirm that they are allocating accommodation based on sex, not gender identity (as per the recommendations in the GQC document).
If they come back saying that they don't recognise the GQC as valid, because it is draft, so won't follow it, that's a bit of a red flag because there is no other available guidance on residential trip accommodation. It depends on how far you want to push it at this point but a "please could you advise which part of the (draft) guidance on residential school trips you aren't happy to follow, when considering the statutory KCSIE guidance" is one way of getting more information. They don't need to comment on the rest of the document, just that section.
Here is the statutory KCSIE document:
https://www.gov.uk/government/publications/keeping-children-safe-in-education--2 (the 2024 pdf - see paras 205-209)
Here is the draft Gender Questioning Children document referenced within it:
https://consult.education.gov.uk/equalities-political-impartiality-anti-bullying-team/gender-questioning-children-proposed-guidance/supporting_documents/Gender%20Questioning%20Children%20%20nonstatutory%20guidance.pdf (I think that's been copied above too but it doesn't hurt to have it twice 😁)
Good luck OP. Hopefully common sense will prevail and they just haven't thought through the question they asked on the form 🤞
If they insist on accommodation by gender identity not sex, it would be reasonable to ask to see their mixed-sex risk assessment on things like sexual assault and pregnancy. Also covering the risk of false accusations of assault. It seems mad that a school/council would want to run a trip with this level of risk.