It may vary from state to state. It is my understanding that, at least in some states, students are required to recite the pledge of allegiance. If so, this is not a matter of free speech. It is a matter of coercing students into expressing particular beliefs.
I am against all such coercion, even without the "under God" phrase.
I completely agree. Believing in God is a choice. Being forced into anything is wrong IMO.