Currently, a teacher doesn't have the right to say those types of things within a public classroom and if they were to do so outside the classroom in a public forum they would probably be fired.
The thing is, using laws to shut these people up doesn't change their views nor does it really do much to stop the spread of their ideas. I also don't agree that people are gullible or that prone to suggestion but rather I know from experience that a person who is compelled to become a racist after going to a KKK or White Nationalist rally was probably already pretty much a bigot beforehand.
The reality is that you can't regulate shitty ideas, bigotry, racism, etc. nor can you really stop such people from spreading their shitty beliefs. Most protests and rallies are people preaching to the converted and by placing a gag on them, all you do in ensure they go underground while also playing the victim.
That said, there are some measures we as a society can take to ensure these types of protests and rallies aren't perpetuating or causing violence nor allowing people to be victimized but such legislation, as I mentioned in another post, would have to be very carefully defined.
Saying something being fireable is deterrent for such behavior is a rather weak solution since school administrators are also capable of sharing those begitrd beliefs, so what motivation would they have to fire a teacher in a town that has poor representation of minorities and there's no legal need to do so?
The idea here isn't to change anyone's belief, but to prevent speech that has a negative effect on societ as well as the victim themselves. You have no solution to this other than to disregard all the offensives and consequences until these movement become so large that they can't be ignored.