There is nothing wrong with ending a sentence with a preposition. The idea that there’s a rule against it is a myth, dating back to a time when English grammarians thought English should conform to Latin grammar (see also “split infinitive”). He must be a bad teacher if he taught them that, and he deserves what he gets. He didn’t even make a “mistake,” as the sentence would have been incredibly awkward if he’d phrased it so as to avoid the terminal preposition — that would have been a mistake.
There is nothing wrong with ending a sentence with a preposition. The idea that there’s a rule against it is a myth, dating back to a time when English grammarians thought English should conform to Latin grammar (see also “split infinitive”). He must be a bad teacher if he taught them that, and he deserves what he gets. He didn’t even make a “mistake,” as the sentence would have been incredibly awkward if he’d phrased it so as to avoid the terminal preposition — that would have been a mistake.