Your comparison makes no sense. If I as a representative of a private company were to badmouth that company, I could expect to be fired. A teacher does not work for a company, a teacher works for you and I i.e., govt. employee... you don’t see the difference?
Now I see what you mean. But shouldn't the teacher be answerable to someone, perhaps to the elected school board?
And I do realize there is a fine line to be drawn here. Whistleblowers are VERY important, and doubly so in public institutions like schools.