Both legal and proper, of course. Quite settled law.
Whether something is legal and "settled" isn't relevant. It's about whether it's a good thing to do. And if it should be legal.
I disagree here. I think the fact that the school's decision to fire this teacher based on the reasons they have given is legal (according to the Supreme Court ruling of January that is mentioned in the article) is very relevant. While it can still be discussed whether such firings are "a good thing" or whether they "should be legal" or not, the obvious legality of this firing should make people who are always ready to shout "scandal!" or "this is another instance of religious extremism!" a bit more careful.