If a couple of close friends chose to share with the trans student- fine. But it shouldn't be forced.
It's not fine! As pp pointed out, a girl and a boy asking to share would be given extremely short shrift, whether in a dorm or a private room, because everybody understands that the adults supervising a school trip have a responsibility to minimise the opportunities for inappropriate behaviour, including sex. And yes, obviously, two lesbian girls could have sex in the dorm on a school trip, but at least there's no risk of pregnancy in that scenario.
The disconnect involved in thinking that it's somehow okay for two unrelated opposite sex children or teenagers to share a room in a residential setting just because one of them identifies as trans is just boggling.