Student was 18? He should have told the principle to cram it up his cram hole, then called the police.
I believe that a better course of action would be to remind the principle that they are legally an adult and legally exercising their 1st Amendment rights. Further, that if she had a problem with that, she should call the police.
That puts the principle in the position of taking an affirmative action to deny the student’s rights and places that action on record. It lays the ground work for further legal action by the student.
Having said that, I also recognize that most teens will not push an school confrontation with authority to the point of involving the police, so it is kind of a tough situation.