Although I usually side with the administration on these kinds of things, the SCOTUS has said “students do not leave their rights at the schoolroom door” (or words to that effect).
Unless they enforce a strict dress code that says, for example “all girls must wear white dresses and all guys must wear blue suits,” then this is totally valid for her to wear and a violation of her 1st Amendment rights.
So, instead of educating students on the true history and meaning of Confederate flag (s) and related imagery, they chose to side with ignorant Leftists who have twisted the proud flag into a lie about racism.