Back when I was in high school, I sat for the pledge to protest the Iraq War. This was in 2002-2003 when people were hyper-patriotic and nationalistic, slightly more than you see today (though still maddening today).
Anyway, I got sent to the principle’s office and so on and so forth, and teachers and administrators wanted to punish me.
However, thanks to a law teacher, I was well educated. If you decide to sit, please keep in mind Tinker vs. Des Moines. That is a Supreme Court law that states students do not relinquish their Constitutional rights when they enter school. This means you have the freedom to protest, as long as it’s peaceful and not disruptive to school, such as wearing an armband or sitting for the pledge.
They cannot force you to stand, and they cannot punish you. That would be unconstitutional. Do not be deterred in any peaceful protest. You are allowed to have your voice, anyone that that says otherwise is full of shit and doesn’t know the law.