Seriously guys,
A teacher getting an observation that says "ineffective" doesn't come from an isolated incident. It's a process by which the administrator observes the growth or lack thereof of the teacher in question. And you know what, they get a performance evaluation like the rest of us in the private sector. All these horror stories of having one bad class or a class in a low SES (socio-economic status) area and getting fired forever are completely baseless.
There are truly terrible teachers in low SES districts, and they are EASY to spot. Principals hold on to good teachers, and get stuck with the ones who are bad because of this moronic tenure system. 3 years and a pulse, and you're in. Stupid.
I can fly an airplane like an ace and make greased landings for 11 months, but if I fail ONE 45 minute checkride with a person I've never seen before, I can very easily lose my job with my current company. Most of us can not withstand a bad performance appraisal, and especially not two. Why are teachers any different?