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.
Anyone with any sanity left knows that the fourteenth amendment was never intended to be the tool for revolution that the left expanded it to be.
Well placed liberals have done their damage and now they pretend it was all on the up and up. It wasn't and it isn't. The men who gave us the fourteeth did not intend for it to outlaw Christianity from school and public life. That was always the radical's goal and they had to find the necessary tool as a pretense for their revolution, the fourteenth amendment was handy and made a good cover story to hide their radical activism from the bench.
After decades of denial that they ignored truth, now they openly admit that truth, history and precedent is not needed since we don't need approval by the American people and the necessary amendments. Afterall, to the radical left, the Constitution is living, breathing, and really has no connection with what it ever meant before. In fact all it really means now is whatever activist liberals want it to mean.