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A safety concern is anything that risks the safety of people. In schools, its anything that risks the safety of the students.

What you are saying is that in this case there are other concerns (self expression) and those trump safety. Thats a reasonable view. However, the school, and many people, don't believe that the school should prioritize self expression over safety. I'm sure many parents don't feel that they should either.

To say that the school's concern is not legitimate is unrealistic. Safety concerns do practice sexism all the time. Its unfortunate, but true. Most people are more scared of a woman walking home drunk at 2:00 AM than a man doing the same.

Edit: Deleted my last line.