Are you really so blind? Of course the school can restrict students. But this is where you should be able to think it through and understand where the battle is raging and why.
It was not until quite recently that a complete rewrite of the traditional meaning of the First Amendment was coerced into the Constitution. This is the battle. As a conservative who actually supports the Constitution and rejects all improper and political assaults to the Constitution, I still reject the leftist revolutionary deconstruction and rewrite of the original understanding of the Constitution.
I hope that sinks in, even into all the irrational and close minded Jacobins who still think anything a leftist court wants to force into the Constitution is progress.
In order to overturn two centuries of understanding and precedent, the liberal judges changed the meaning of the first amendment and then forced it upon America against their will. This is the battle, do you understand yet.
The battle is against the liberal and radical revisers who intimidate any and all school districts to ignore American history and the Constitution and instead obey the modern radical viewpoint of the first amendment which stemmed from anti-American radicals such as inhabit left wing cesspools like the ACLU.
Before the radical humanistic rewrite of the First Amendment, children could have voluntary group prayer in school, voluntary Bible readings, a view of reality that included a creator God , and the textbooks often referred to the faith of some of our greatest citizens and leaders. Also, before the humanist rewrite, teachers never promoted the acceptance of abominations such homosexuality, abortion and other vile depravities.
Once the incredibly biased and treasoness rewrite of the First Amendment was driven home by social engineering collectives like the ACLU and others, they can and do fanatically enslave all public school children and the parents who pay for it, to accept the revision as if it were fair and just. It has become the leftist's hammer.
Some of us were alive when the ACLU view was so far left that they only whispered it in their drug parties and whore houses. Their view is still insane, but now the American people have undergone quite a few years of radical reconditioning. You are ignorant of American history and the methods and lies of the ACLU and the Constitution breaking court. I will never accept their nonsense, but the left has the power to indoctrinate by law, and they use it by force every day of the week, just like the old Soviet Union.
Parents have had their influence taken away, unless they have bought the perverse views of the radicals. Radicals are the ones who say what goes in the modern public school, not parents.
If parents have bought the politically correct view that you push, that only means that they have been pushed to accept a radical unConstitutional rewrite.
If the school in question were a private Jewish school, IMO, they would have the right which this school is claiming. They would teach that Jesus was NOT the Messiah of Israel, and that he either did not exist at all, or was a fraud. They would have the right to proscribe speech which tended to impugn or discredit the religious purpose of the school.
But this was not a yeshiva, this was a US public school, open to all including Christians.
And the right this girl was trying to use is guaranteed by the First Amendment, as expanded to the States through the Fourteenth Amendment, and that right is the Free Exercise of Religion.
You are right that Everson and its progeny have, by using a blatently false definition of "Establishment of Religion", succeeded in extinguishing Free Exercise.
But the Constitution cannot contradict itself in this way.
If a new definition of "establishment" bans "free exercise", then the problem is not with either disestablishment OR free exercise, but with the new definition.