If you have 20 teachers who are fair, it would not be surprising for a statistical analysis to show that one of them is 95% likely to be unfair.
Quite. If I was in a position to be firing people, though, I'd be shooting for considerably more than 95% confidence -- a level of confidence that should be achievable with the kind of data sets that the OP was talking about.
Post will be returning in Main, after a rewrite by the company's writing staff. Citations Galore.