That would be for the courts to decide, not the kid who decided to break the rule, then complained of the consequences.
Exactly. There is no hero and villain here - both the principal and the student are wrong. According to the apparent school policy, and state law, the principal broke both of them, and is out of control, but whether the rule the principal created or the principal's actions were constitutional or not is a separate issue from what the student did.
The student knew he didn't have to have his speech pre-approved, either according to state law or to school policy, yet he agreed to have it approved, anyway. It was at that time that he should have taken his stand. Instead, he agreed to the censorship, and then promptly went back on his word.
Whether or not the rule was constitutional is irrelevant to the kid breaking his word. Whether he is 17 years old or 18 years old is also irrelevant. The message he gave is irrelevant - it could have been about God, the devil, Communism, unions or baseball, hot dogs, apple pie and Chevrolet, it doesn't matter. The kid deviated from what he agreed to. What matters is his word can't be trusted, because he thinks he can ignore the rules and break his promise whenever he feels like it.
Is he the next Bradley Manning in-training, only this time in a Naval officer's uniform?