The guesstimate that struck down California’s teacher tenure laws.
…testified that 1 to 3% of teachers in California are grossly ineffective. Given that the evidence showed roughly 275,000 active teachers in this state, the extrapolated number of grossly ineffective teachers ranges from 2,750 to 8,250.
The expert cited in the ruling doesn’t even necessarily believe that low test scores qualify somebody as a bad teacher. Read this article in its entirety at Slate.com