Here are a couple more scenarios you could chuck at your Head while you're at it to illustrate the safeguarding and legal risks of allowing mixed sex accommodation on a 'case by case' basis. Letting something to go ahead because 'this child is lovely' is going to cause all kinds of problems down the line. Crucially, no-one has to actually 'misbehave' for really unpleasant and upsetting things to happen. But given the known high levels of sexual harassment and assault in schools, it seems fabulously naive to assume that nothing bad could happen.
Child E, 14 is male and declares a trans identity and, following a change of haircut, name and clothes is allowed to share with girls on a school trip. Two girls agree to share because they like E and want to be inclusive. E has a visible erection in his sleep, having kicked off the duvet in the night. Girl F thinks this means that E is masturbating and complains to the teacher. E wakes up, totally unaware that anything has happened, to find two teachers and two crying girls in the room, and being accused of being a pervert. F calls her parents, who call the police. F's parents sue the school for having violated her rights to privacy and dignity. E's parents make a formal complaint of transphobic bullying.
I, 15 is a male child, who identifies as a girl and is attracted to boys. I changes name, pronouns and dress and is allowed to use the girls facilities. I is generally well liked by the girls, and no-one voices any objection. Subsequently, J, a male child who is already sexually active with girls, and not at all well-liked, announces a trans identity. Some of the girls express misgivings in private to teachers, but nothing concrete, so J is allowed access to girls facilities based on precedent, as there is no firm basis on which to deny this to J. A school trip is planned:
a/ None of the girls are willing to share with J so J is offered a choice of sharing with boys or private accommodation. J's parents make a complaint of discrimination for not providing some girls for J to share with, when that's been done for I.
b/ The school allocates some girls to share with J, without offering an opt out. The girls refuse to go on the trip. Their parents complain that they've been excluded from the trip, and that their right to privacy and dignity has been violated.
c/ The school coerces the girls by saying that the trip can only go ahead if some of them volunteer to share with J. Three of them agree, but refuse to be left alone with J, and give J the cold shoulder during the trip. J's parents make a formal complaint that the school has failed to protect J from transphobic bullying.
d/ As above, three girls agree to share with J. Following the trip, one of them tells a teacher that she woke up and J was standing over her bed staring at her, and some of her underwear is missing. Her parents make a formal complaint. J denies everything, then claims to have got a bit lost going to the toilet in the night. The girl’s parents sue the school for putting their daughter at risk. J’s parents sue the school for discrimination, saying ‘no other girl’ would have been asked the same questions.
e/ After the room allocations are made, two girls approach the teacher to say that J's behaviour in the toilets and changing rooms has been making them uncomfortable. 'It's not that big a deal and we didn't like to say anything because of course J is a girl, but now that we're going to have to share a bedroom...' None of it is very concrete, so the teacher makes the decision not to do anything. J makes sexual remarks to the girls in the bedroom during the trip, and their parents make a formal complaint to the school. J says they were jokes and makes a complaint about transphobic bullying.
f/ I shares a room with girls without incident. A year later, I reverts to his original name, pronouns and stops wearing makeup. The girls are distressed that they were persuaded to share a room with I because they were told by the school that I was a girl. They make a formal complaint to the school for failing to protect their privacy and dignity.