"The particular leaflets in question were not advocating violence; they were advocating membership in a Wiccan group."
Totally beside the point and nonresponsive to the questions that I raise. Do you consider the ability to limit the free speech of students by school administrators to be absolute?
"Minor children do not have "free speech rights." As pointed out above, schools regularly limit the "free expression" of students. They're students, they're children. Children are subject to parents and school administrators."
And this girl is a minor? She's a graduating senior and probably 18. So, it's really sounding like, in your opinion, a school administrator's right to silence anything a student wants to say is absolute and unchallengable. That's pretty frightening, but it would prepare them for life in the kind of world such thinking would lead us to. "Shut up, say what I tell you to, and don't think for yourself."
Since children don't have free speech rights, I reject the premise of your question.
However, school administrators act "in loco parentis," and courts have recognized that they enjoy the same ability to limit children when they are at school that you do when they are at home.
She's a graduating senior and probably 18.
She is still under the jurisdiction of the school. When she gets her diploma, she is free to do and say whatever she wants.
That's pretty frightening, but it would prepare them for life in the kind of world such thinking would lead us to. "Shut up, say what I tell you to, and don't think for yourself."
If she enters the world of the military, she will find that she had more "rights" when she was a student. And, depending on where she chooses to work, she may find that some companies will limit her right to speak about certain things or to proselytize on company property.
Even as an adult, one may exercise one's right to free speech, but there may be consequences if one does.