Without knowing the details, this case is hard to judge. Defamation is not protected by free speech and this sounds like defamation against Muhammad. I also think that defamation of a religion is indirect defamation of members of the religion. And if this teacher taught in a publicly funded school, that makes this even worse. Note that I am not Muslim and that I strongly support free speech. Before America became a leftist cesspool, it had my ideal of free speech and that included state anti-blasphemy laws with the understanding that blasphemy is simply defamation of religion.