In this day and age of everythign being risk assessed to the n-th degree, it will be interesting to see how the risk assessments are done for these scenarios, particularly the sharing rooms on school trips. And particularly if parents raise the issue in advance, state their concerns in risk assessment terms and ask for reassurances that their dc (and other dc) will not be subject to the risks...
Unfortunately in many of these situations it's money that talks - the risks of being sued as the result of something happening that has been raised by parents in advance, but not covered properly in the risk assessment and then not acted upon properly could leave the school open to being sued and paying out lots of money they don't have.
Hate to suggest it but maybe one way to deal with this is to ensure that schools are aware of parents being aware of this and not wanting to be put in a position where they could be sued. Downside I guess is maybe that the parents/trans child might also be jumping up and down and threatening the same and the trip gets cancelled.
Part of the problem for me is that it's such a big thing but treated so cavalierly. It's easy to envisage situations where a boy says that he's currently identifying as female on a trip so that he can sleep in the girls rooms to be with his girlfriend or because he thinks it will be cool or because he wants to attack someone or a host of other reasons that are not genuine, but using it as a means to an end. For things like this it would be a start if there was some sort of formal process - so that you would have to formally register and have been registered in order to benefit from the change. Doesn't need to be with the doctor - no idea who it should be with though, maybe the local Registry office? - or for as long as is needed currently. But something that is a definite thing.
In the same sort of way that If people want to change their names they can - but as soon as they want to have official recognition of that name (for a passport or driving license or whatever) then they need to do it formally by deed poll. Small fee, name becomes official. It's quite a big deal to many people to change their name for a host of reasons but it's not as big a deal in some ways as changing sex (particularly having drugs and operations) and yet it seems there's going to be no way of recognising the changes . Maybe having some sort of formal trans deed poll thingy would help to filter out some of the piss takers and then at least schools could say that only those that had done the poll thingy could be recognised as such for school trips or school sports etc. WOuldn't solve everything on either side by a long shot - but I think it would be better than it is now. It wouldn't be forever permanent in the way that an ordinary deed poll isn't - just until the next time you change your name by deed poll, it's not like you're only allowed one shot at it. It could also help provide a useful talking point between the different groups, even if a different solution is found eventually.
Sorry, that's a bit waffly, bit tired to think concisely! Hope it sort of makes sense though.