Grammar and usage are two different things, and understanding the difference can be a huge help in untying some linguistic knots and navigating some linguistic swamps of the English kind. Because the "grammar police" are almost never policing grammar; they're enforcing something else entirely. And yes-- if you stick around for this, you'll get some of the same stuff my students did for years.
Grammar is the mechanics of language. Or at least, the best model we can come up with. Because here's the thing-- we don't really know how the brain does language. We know that there's a readiness factor (that would be why you learn your first language when you can't even dress yourself, but trying to learn a second language in your teens is twelve kinds of torture). Language is a big black box in the brain, and we don't know what's going on inside it.
But we can reverse engineer it. If you played Pacman, you learned a bunch of rules to the game--which way to turn, when to turn, where to pause, etc. You did not learn those by printing out and studying the program for the game. You learn by carefully observing what works and what doesn't. There are plenty of other examples-- we don't actually know how/why gravity works, but we can accurately describe how it will behave.
So to for grammar. Grammar rules are descriptive, like Newton's Laws of Motion. When we say that every sentence must contain a subject and a verb, that's not because somebody decided it would be a good idea, but because we can observe that when a sentence lacks either a subject or a verb, it doesn't work. Traditional grammar, like Newtonian physics, mostly describes things pretty well, but has some real holes in it. There are other grammars out there, but none have ever really caught on.
Grammar is all about the mechanics of how language works, what it parts are, how they function. Traditional grammar is pretty mechanical and not too pre-occupied with meaning (which is a problem that kind of seeps into language instruction in general from there, but let's save that topic for another day).
Mostly, when you break grammar rules, people look at you funny because you don't make sense. Sometimes, they look surprised because you make sense in new and interesting ways (lookin' at you, Will Shakespeare).
But the people you meet online who are called "grammar police"? They aren't really grammar police at all.
They're usage police.
Usage is about the "right" way to say something. It's what word is the "proper" word to use. It's what people judge you for, what they want to correct, what let's the words "right" and "wrong" into the conversation. All its rules are prescriptive, like a school dress code.
Usage is fashion for language. And like fashion, it doesn't always make sense (at this point, I would always point out to my students that we could all tell I was the most dressed up person in the room, because in the morning I took a special piece of fabric and tied it around my neck in a special way, and why is that even a thing, and by the way, think about the phrase "dressed up"). And like fashion, it is set in ways that are a little bit beyond human control.
For instance. Lots of languages have plural forms of "you." English did, too, once upon a time, but then we just sort of dropped it, but because it was useful, we eventually kind of put it back-- y'all, you guys, or, in my neck of the woods, y'uns.
For instance. Look at the enormous amount of effort it's taking to sell the replacement of "he" or "she" with "they." "They" offers a real solution to a real problem--the old rule that "he" was always the default when you had a singular person of unknown identity--but it's really hard to get language to change just because you want it to.
Usage is also the part of language where we start talking about meaning, both text and subtext.
So grammar is the attempt to describe what's happening in a part of our brains we cannot see or control. Usage is a social behavior, a linguistic fashion that responds to a variety of factors.
Classic example-- "ain't." "Ain't" is perfectly grammatical. Native speakers understand it, and they can recognize when it's used in a way that makes sense (That ain't it) and when it's not (Do you like my ain't hat). But ain't runs afoul of usage rules; it's not widely accepted as "proper" English, by which we mean--and here's an important part--the English used by people who have social status.
Another big difference is that usage changes, and grammar mostly doesn't. A sentence has required a subject and a verb for centuries. But like fashion, usage shifts. Sometimes we shift to something less formal (just like all the politicians who now campaign without a tie), and we are constantly experiencing usage "fads," where words or phrases erupt, spread, subside.
That shifting matters extra to English.
The trouble with English
Our language suffers from history. Specifically, the history of Britain, specifically their unfortunate tendency to be invaded a lot.
It's fair, because the language starts with an invasion. Those Anglo-Saxons attack and take over the island nation, and at this point, we tag the start of Old English (Angle-ish), a language that a modern English speaker can't understand. Then along come the Danes, and the big language-buster, the Norman Conquest. After the conquest, most of England's upper class speak French-ish, and Anglo-Saxon is spoken mostly by the vulgar lower classes. It's a social fashion that affects us to this day (or did you think there was some good reason that "shit" is a naughty word but "defecate" is all sciency and smart). And then of course the language gets spread to the US, where we are just melting people into the pot right and left. End result is that while some languages tend to be kind of stiff and set in their ways, English is flexy and bendy.
When usage started to be codified, folks mostly just codified the same social status and fashion that had always been at the heart of it. Who says this "correctly"? Must be the rich upper-crust folks of London, and not those poor hicks from the hinterlands.
This process has never stopped, and it has never, ever been divorced from class and status. Have we grown a bunch of different languages? Probably not-- there's plenty of academic argument about this sort of thing, but the short layperson answer is that if someone is talking to you and you understand them, then the two of you speak the same language. But we're loaded with dialects and accents and versions of English, and there is absolutely no objective method for determining which is "better," just like there's no objective method for determining which pair of pants is "best."
"Correct" usage is always a social construct, and context always matters.
Deliberate language change is hard
Is correct English a white thing? Sure. Also a classist thing. And knowing that is important. But changing it is tough. When you see a demanding statement like this NCTE group demanding linguistic justice, it's hard to imagine how to make it work.
For one thing, they question if white students are asked to code-switch and drop their native usage patterns, and as someone who taught many low-income white kids, the answer is yes. Shaking the language you grow up with is hard; your tribe tends to push out other usage patterns. A country boy from my region may be looked down on if he walks into a swanky country club in the Hamptons, but if those folks walk into an auto parts store in my town, they may well get the same treatment.
For another thing, making deliberate usage rule changes is as hard as making a particular fashion trend happen on purpose, or getting a video to go viral. Hell, it's so difficult that some of us made entire careers that were at least partly about trying to coach those transformations.
You can wear a floofy purple hat in public every day to try to make floofy purple hats happen, but at some point, you may decide that it will help you achieve some of your goals to leave the hat at home. The argument about non-standard English has raged forever--on one side is "It's insulting and oppressive to tell my child she speaks incorrectly" and on the other side "I expect you to give my child the tools she needs to have mobility and achieve her goals."
It is possible to do both. And what we can most definitely do is understand that usage rules are not sent down from the Mount on stone tablets. They are not inviolate and they are not a solid means to judging someone's intellect, value or ability. And for gatekeepers like teachers and professors, there is always a need to look in the mirror and ask, "Am I just putting a lot of weight on mechanics and usage because grading content and expression is wiggly and hard?" Because you can definitely stop that.
In the meantime, it's fair to respond to grammar police with statements like "On whose authority do you believe that's correct." If they actually call themselves grammar police, you ask them to explain what they think grammar is and where it comes from. And you can always just say, "I ain't got time for that shit." Which is totally grammatically correct.