Well a quick Google tells me it's not that common, and children with this will be under fairly close medical supervision.
As such, the school would be well aware.
And it would be up to the child, and maybe what the school and family think best, where she changed. I'm guessing a separate area would be a solution. Or they might be excused from swimming if they were really uncomfortable.
Separate facilities have been rejected as a good option for trans children (and adults) again and again, unless they prefer that. That is the same with loads of stuff.
Single sex changing (when communal) is hideous enough. For both boys and girls. Mixing the sexes is not on.
And FYI using girls with DSDs, lesbians, the USA history of segregation to support your ideas is a dick move.
And a distraction.
Focus on the group you are talking about.
Explain why males with fully functioning bodies should be considered as girls or women. Without co-opting the situation/ struggles/ history of other groups of women. Can you give that a try please.