[Elsie]

..., first-grade girls must share their bathroom with a fellow first-grader who has a penis.

Just like they do at home if they have a BROTHER!

[generally]

Most brothers and sisters are not in the bathroom simultaneously. At least not beyond a very young age. By the time most kids are in first grade, if they have opposite sex siblings, they are taking turns in the shared home bathroom, not using it simultaneously.