Use of "their" in lessons for singular to be PC


I think this "their" thing is stupid. If someone is not sure of another person's gender they should use "he/she" not "their."
Maybe it's a regional thing, but where I come from using 'their' to refer to just one person is perfectly normal, and as already mentioned, using it to refer to one person is actually gramatically correct.
A quick google search shows that the definition of 'their' is: (used with a singular indefinite pronoun or singular noun antecedent inplace of the definite masculine his or the definite feminine her):

These days, singular "they" is considered to be grammatically correct. It's endorsed by the style guides of major publications such as the Washington Post. Indeed, singular "they" was the American Dialect Society's word of the year in 2015. Historically, it was used by authors ranging from Shakespeare to Jane Austen. It wasn't until the 1800's that some grammarians got together and tried to set a different standard.
Thankfully, we're finally getting over that. Who really wants to use clunky phrases like "he or she" anyway?
https://www.youtube.com/watch?v=f21t7DRKlg8

There will always be grammar pedants.
'he/she' and 'his/hers' are clunky. 'It' and 'its' are demeaning. There are handfuls of new pronouns proposed, but people can't agree on one, much less push one to be used in the mainstream. 'they' and 'their' are there and already in use, and I can't recall once being confused by context.
"John played their bishop..." is fine if we have reason to doubt John's gender identity. When we don't, we use 'he'. Likewise, if we don't know who's playing their bishop at all, we do what I just did there until we find out it's none other than John himself.
Pronoun reappropriation has happened several times before in other languages, and I'm sure their language pedants got their cotton twisted too, but they got over it and let the living organism known as language do its thing.

Interesting thread.
As a native Portuguese language, I tend to agree to MArtin's post.
Usage of their as a gender neutral pronoun is an accepted grammatical device in English.

I sympathize with the OP; he or she expressed his or her viewpoint quite clearly. I just hope that he or she will not find himself or herself targeted by the politically correct crowds.
Sorry, could not resist...

The simplest way is to use "the". "White moved the knight to C3", to me, is way better than "White moved their knight to C3". [team chess? communal piece?] To me, "their" is cringeworthy each time I read it. Second best, as many pubs do, is use his in half the lessons and her in the other half. I'm male, I wouldn't care if they use her in every lesson.

Use of "their" to refer to just one person, as a way of avoiding the ugly his/hers construction may or may not be grammatical, but it is widely used, widely understood, and it is no use complaining about it (or merely saying "I don't like it") because that is not going to make it go away.
I disapprove of the loss of many useful words. For example "enormity" these days is rarely used to mean anything other than big, and to say that "we had a gay time", which once meant simply that we had had fun now produces schoolboy smirks and giggles. We have lost shades of meaning in many more words than those, but there is nothing I can do about it.
Usage changes. There are better ways to spend ones life than railing against the inevitable.
I think Mr. Covey had it right in his ideas of "circle of influence" and "circle of concern" even if he spoiled his great book by trying to recruit new followers to Christianity alongside describing the seven habits.

The simplest way is to use "the". "White moved the knight to C3", to me, is way better than "White moved their knight to C3". [team chess? communal piece?] To me, "their" is cringeworthy each time I read it. Second best, as many pubs do, is use his in half the lessons and her in the other half. I'm male, I wouldn't care if they use her in every lesson.
Your proposal is that instead of altering the aesthetics of the language to reflect modern usage, we should instead damage the functionality of it in a way for which there's no trending precedent.
Did you think of this switch while you were brushing the teeth?

Are you kidding me? You think "their" is aesthetically pleasing? Using their is not modern usage, it's just grammatically wrong. You can have an excuse for anything.
Why woud anyone want to be PC? The correct PC is to refer to all as "it". That way you cannot produce a "microagression" for it by accidentally assuming a gender identity for it or implying that it has a race or something like that. The best PC plural version is debatable, but any generic term like "beings" or "entities" will do so long as the term has no gender or racial implications. Naturally this produces stupid sounding english, but that is OK for PC; I think it might actually be the POINT of PC. This works. After about a week of refering to an annoying PC police person as "it", they usually go away and leave you to speak correct english in peace.

Neither Shakespeare nor Jane Austen would write the sorts of sentences that have become common today, but it is true that both used "their" in the singular on occasion.
Years ago, my youngest son came home from kindergarten (he's in his mid-20s now) with a note that said, "When your child arrives at school have them put their coat in their locker." That sort of writing exhibits laziness, not the necessity of using "their" as a gender neutral singular pronoun. Citing Jane Austen in defense of such garbage fills my mouth with bile.

You can always learn a language where you can omit pronouns and it still makes sense, like Japanese or Spanish. Besides that, get used to being annoyed with English.

While I am sympathetic to your cause, OP, I find it hard to ally myself with anyone that argues for grammatic correctness, then uses the phrase "way better."
The simplest way is to use "the". "White moved the knight to C3", to me, is way better than "White moved their knight to C3". [team chess? communal piece?] To me, "their" is cringeworthy each time I read it. Second best, as many pubs do, is use his in half the lessons and her in the other half. I'm male, I wouldn't care if they use her in every lesson.
White moved their knight, or their opponent's?
"White moved the knight to C3" is merely a verbose way of communicating "Nc3".