Let's say that we're very confident in our ability to recruit and retain teachers who are better than our current teaching force and so we decide to fire all below average teachers (a full 50%) -- which would be a far more aggressive plan than any I've seen proposed. First, the majority of these below average teachers are novices who are still improving and for whom we don't have particularly good estimates of ability. Given that the majority of struggling beginning teachers either improve or self-select out of the profession, let's estimate that 2/3 of all teachers in their first 5 years are identified as below average teachers. This would mean that only 1/6 of all teachers in their sixth year and beyond are below average teachers. And since only 1/3 of teachers are in their sixth year or beyond, this would mean that only 1/18 of all teachers would both have 5+ years of experience and be rated below average. This is a little under 6% of all teachers.
The average school in my sample had 72 teachers. So, that's the equivalent of firing four teachers. And that's under an extremely aggressive scenario. Besides, now that you've rid your school of the chaff, who, exactly, do you want to fire next year? And if you want to argue that we could be more aggressive and fire some of the novice teachers, that would mean there'd be fewer low-performing experienced teachers (since teachers tend to be roughly equally effective pre- and post-tenure). So, for now, let's stick with the assumption that, under an aggressive plan, we'd fire four teachers this year in the average school.
Now, other districts have far more experienced teachers. And it might make more of a dent there. But a good number of our poorest-performing schools and districts are quickly churning through teachers too fast for firing low performers to make much of an impact. Certainly, we should make every effort to rid our schools of the worst teachers (by increasing the performance of, and/or dismissing the lowest performers) -- I don't think anybody seriously disputes that notion. Or, at the very least, I don't think anybody serious disputes that notion. But will firing the lowest performing 6% of teachers in high-poverty NYC schools make a difference? It's possible. But let's be reasonable -- it's not going to make much of a difference.
If I had my own foundation, I'd fund the creation of an agent-based simulation of teacher hiring and evaluation practices.