"Listen, I don’t have a problem with firing teachers. I just don’t think the common perception of “infinite job security” for teachers is accurate.

A much larger and more concerning issue - in my opinion - is that we have very poor measures for teacher effectiveness."

If bad teachers, by your own admission, are not being fired, teachers have the very job security you claim they lack. If being fired for bad performance is unlikely and schools are much less likely to run out of funding than a company (if run even semi-responsibly), these jobs may not have "infinite job security" but they're as close to that as can reasonably be found. After all, what is job security if not confidence you will not be fired?