Why not? School grounds are not normally open to the public, and non-students usually have to be invited to be there. And on just about every ticket I've ever seen, whomever is inviting the person reserves the right to enforce its rules, and to eject anyone who doesn't comply.
In other words she did not have to wear the training burqua because it could cause her physical or emotional damage or (under title six) if she had a religious objection to it.
It is the same if she had brought her seeing eye dog and they tried to arrest her. Or if she was wearing a burqua and they told her to uncover her face.
They can make rules but the rules always have exceptions built in for people who can not comply. Especially in public facilities.
Both the police officer and the school know that. They just wanted to throw their massive weight around. So they will be on the receiving end of a massive lawsuit.