Curriculum itself can be written to include "incorrect, misleading or incomplete language" so your argument isn't quite the slam dunk you think it is.
Assume a state adopted a curriculum that some say is promoting Southern "Lost Cause" mythology. Would a teacher have the right to alter his language to avoid teaching it?
Either teachers can teach what they like (i.e., freedom of speech/religious expression) or they are peons only able to regurgitate state-mandated curricula. I think you'd like to have it both ways.
I don't want it both ways. I want teachers to teach the truth. The truth exists independently of what the state government or they personally think.
If the school curriculum is based in reality, then the teachers should stick to it.
If the school ciriculum is not based in reality, then the teacher should teach the truth.