I suppose I should have said it's doubtful that you lived at a time when the argument against singular they was legitimate. I'm sure many teachers are teaching

I suppose I should have said it's doubtful that you lived at a time when the argument against singular they was legitimate. I'm sure many teachers are teaching students incorrect things all the time. But by modern English standards, which I would define through works like Fowlers', the legitimate argument against singular they is long over.