For right now I dont think it is a double standard.
She has a right to religious expression, just as I do. As you pointed out, their is nothing keeping me from praying on my own, wearing religious tshirts, crosses etc within the confine of the dress code, I just cant lead a class in a religious ceremony (unless something has changed since I was in high school). Thats because I would be imposing the practice of my religion on others.
It would be a double standard if she were to say make every women in her classroom wear a scarf, forcing others to pray 5 times a day, etc. But she is not imposing her belief that all women need to be covered on others. She is only imposing it upon herself.
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Or kippot for Jewish students?
They should allow kippot's, as well as any other religious article that is deemed necessary for a person to be considered a good faith practitioner of their faith.
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Are they going to have to allow WWJD hats now too?
I would argue the wwjd hat would really not fall into that category because it is not necessary for the practitioner of christianity to be considered a good faith practitioner of their faith.
The headwear rule would fall apart if say some gang declares themselves a religion.