The Free Speech Clause of the First Amendment protects your expression. But, the First Amendment also has an Establishment Clause, which bars any government actor, like your public school, from promoting an establishment of religion. In general, schools have wide authority to shut down speech solely because it is religious. Whether they can shut down speech under the Establishment Clause or if it is protected under the Free Speech Clause depends on two questions: (1) whether the students hearing the message are a “captive audience,” and (2) whether the student’s statement constitutes religious speech.

As long as you aren’t causing a substantial disruption to the game, kneeling during the anthem is protected by the First Amendment.