Your rights to free speech at school is not unlimited or absolute. Your access to the school for a recreational event (non-educational purposes) is subject to time, place and manner restrictions. You can raise a legal challenge to individual decisions by the school administration, but those are generally brought before the event. It is always an uphill battle to persuade a court to overrule a school's limitations on student conduct, but it can be done and some very significant court decisions have come out of such factual circumstances.
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