That's exactly what I am talking about :-) In my idiolect, it's a he; in your idiolect, it's a they; our idiolects are perfectly understandable to both of us; and yet, instead of simply recognising a variation in my speech, you feel an urge to change my language.

Remarkably, it's exactly the opposite of the public attitude towards regional dialects. Where previously it was common to correct speakers of non-prestigious dialects, it is now required to be accepting and tolerant.

I take issue with “perfectly understandable”.

I think many (most?) people hearing or reading that would make an assumption from your use of ‘he’ that you expect that professors and students are usually male. Perhaps that you believe they _should_ be male. If you really mean them to be gender-neutral, you are not communicating clearly. I would argue that this has always been true - but even if I’m wrong about that, it’s true for modern English usage.

The hypothetical "someone" in my sentence (a professor or a student) is indeed probably male if I look at him closely. I pass no comment on whether professors or students _should_ generally be male; but at the same time, have no qualms with imagining them as such. I understand that many think differently, and have seen plenty of texts where an imaginary character of unspecified sex is introduced with a third-person singular feminine pronoun (a random example: "For a Product Owner to properly adapt a product, she needs some empirical evidence, something to inspect", from "The Professional Product Owner" by Don McGreal and Ralph Jocham). My argument is that if should be perfectly fine for a writer to use the default gender that he is most comfortable with.