I'm not going to argue that there aren't bad teachers or there aren't a LOT of bad teachers. But IMHO a lot of what I have seen at my company is the following:

If a kid brings home a bad report card - Parents often blame the teacher. Obviously, their kid is great, the only reason they are struggling is because of negligent or poor instruction.

If a kid brings home a behavior citation (i.e. teacher/administration called home and said their kid was poorly behaved) - Parents may or may not recognize themselves as a contributor to that poor behavior.

But for the teacher, it doesn't really matter. If there is one poorly behaved kid in the class, it's going to make it harder for that teacher to instruct every single kid.

It often seems that the problem isn't a glut of "bad teachers", it's that the structures placed around the teachers all too often seem to be set up for them to fail. It's no surprise then that 40%-50% of teachers quit the profession entirely within five years http://www.edweek.org/ew/articles/2012/05/16/kappan_ingersol...