This is NOT freedom of speech. Places/people/things have the right to treat you or act based on what you say. Example school wants you to say the pledge of allegiance and you decide to sing I'm a little teacup instead. So they right you up/give you detention, you cant then claim freedom speech.

The only thing the constitution protects you against is the government making laws against speech and peaceful assembly aka you can be arrested and prosecuted for it does not mean you can say w/e you want or not say something and have no consequences.

(Im assuming OP is in school) If OP refuses to say in "under god" and the school decides to give him detention for it they are technically allowed too. Would it be correct of them? No but that's their right aswell.