Not every profession is knowledge-based.  A worker at an assembly line only needs to know where to attach the part assigned to him, and a taxi driver hardly needs to know more than the roads in and around his town. They may know nothing else, and still be excellent at their jobs. 

But how about people whose job it is to pass knowledge to others -- the teachers? Should their own knowledge be adequate? Is there a problem when teachers don't know what they are talking about?

I think that would be a problem -- for a simple reason that teachers are authority figures to the students. Student's tacit acknowledgment that a teacher knows more, and hence needs to be listened to, is the cornerstone of the profession. Teaching is based on a respect for teachers' superior knowledge. Without it, no teaching is possible

So what happens when teachers are wrong in what they are teaching? How does it affect students? 

I never gave it a thought until reading this news article, "San Diego teachers’ Union passes resolution rejecting Israel’s legitimacy" The resolution bemoans creation of modern Israel and completely ignores Palestinian terrorism.

So what can those teachers teach their students? That Jewish presence in “historic Palestine” is illegitimate? Yet, that flies in the face of all historical data, which tells us that the area was Jewish for about fifteen hundred years, from the exodus around 1200 BC till the Roman expulsion following Jewish rebellions, the last one lead by Shimon Bar Kokhba. Nor are Arabs native to the land -- they attained their ownership of it in 636 AD in the wave of Arab conquests in which they gained possession of the half of the then-known world, from Spain in the West to the border of India in the East. Hence, teachers' whitewashing of Palestinian terrorism (by simply not mentioning it, apparently on the grounds that it is a legitimate manifestation of a liberation movement against Jewish occupiers) displays their monumental ignorance of what they are talking about.

That it is not particularly helpful for the blind to be led the blind has been known for a very long time -- for a couple of millennia, in fact. One may similarly ask, does being taught by the ignorant lead one to become educated? What is the use of imbibing falsehoods as facts? Where will the distorted view of reality lead one?

The news report I referred to, argues that the result will be hostility and hate towards the Jews. In the ignorant Middle Ages, and in the Nazi Germany that idolized and imitated them, it produced some very ugly results. Ignorance is not good,and spreaders of ignorance can hardly be called teachers.

Yet for some odd reason, the authors and supporters of the "San Diego teachers’ Union resolution" do think of themselves as "teachers." This is a mistake. Ignoramuses cannot be teachers. "San Diego ignoramuses" is a much more accurate moniker for them.

Which leads to the question with which this discussion started -- why does San Diego allows ignoramuses teach the children? What can the ignorant impart to the young and inexperienced, except for errors and lies? What is the use of such "education"? Why do those frauds call themselves "teachers" -- and why are they hired and paid by the city of San Diego, as if they were?