I am a former teacher and professor who taught teachers. Teachers are like everyone else - some very smart - and some just smart - and some very UN-smart.
Add to that formula the horrible mixing of classes full of all the above - in one room. You have our public school system. (further add into the equation more than a few chemically "dumbed-out" - and there is little to no learning actually taking place. Many teachers have retreated into that "safe place" inside the canyons of their minds. They would rather many students were already in prisons or, at least, not in the classroom.
http://dennishooker.com (ask for any of my pdfs at no cost)