Nothing in Freedom of Speech releases the speaker from consequences. It is true that the government may not limit free speech beyond the obvious limits of slander, incitement, etc. but other hearers may take legitimate action to the disadvantage of the speaker.
The government isn't censoring the teacher but giving him guidance and direction and correction regarding improper or inappropriate material in his classroom.
His employer, the school, has every right AND DUTY to ensure that his classes are taught properly or as they see fit.
He went out of bounds and was brought back in.
No one is saying that he can't rant about Bush being like HItler and this isn't a Free Speech case. On his own time he can say anything he pleases.