The U.S. public school system has a genuinely weird hatred for grammar. I went through secondary school in the ‘00s, and we got extremely minimal grammar instruction paired with a lot of comments like “y’all are so lucky i’m not having you diagram sentences, i had to do that in school and it was just the worst.” In college I did learn how to diagram sentences, and not only is it not nearly as unpleasant as I was led to believe, it’s actually really helpful in understanding how sentences are structured.
Later, when I spent a year teaching 11th and 12th grade English, I eventually broke out the sentence diagrams because it was clear most of my students had no understanding of sentence structure and it was screwing up their writing. (The most pervasive problem was subject-verb agreement, because they couldn’t identify which nouns were supposed to go with which verbs: they had a habit of writing things like “the rooms of the house is” or “the tree full of apples are” because they made their verbs agree with whatever noun was closest and didn’t get why that would not always be correct.)
And, sure, it’s a valuable thing to normalize dialects of English. The idea that one dialect is “proper” English and all the others are somehow wrong is not only classist and racist, but it’s just bullshit, linguistically speaking. However, code-switching into formal English is nevertheless a valuable skill — even if we don’t teach grammar in schools, your potential employers will still judge you for it — so you should learn the rules.
When I teach Introductory English to college freshmen, I generally split the difference: I tell them that I’m not going to take points off for grammar, but I will make comments on their paper pointing out where their writing differs from Standard American English. And we have multiple readings & discussions about dialect and the fact that no dialect is inherently more “correct” than any other — which, I’d like to point out, I never got in high school, despite the ostensible de-snobbification. The impression I got instead as a teenager was, “there is a single correct version of English, we just refuse to teach it to you because it’s boring.”
I am still skeptical about the assertion I often see that not teaching students to write cursive means they won’t know how to read cursive — those are two different skills. I knew how to read cursive in kindergarten, because I had a picture book where the words were printed in a cursive font, and I figured it out. It’s not hard; most of the letters look pretty much the same. I got some minimal instruction in how to write cursive in second grade, but I was never very good at it and I couldn’t do it now.* Still read it fine, though. I don’t really see that reading cursive is a skill distinct from reading, say, messy handwriting or unusual typefaces.
*Anecdote: During my last year of high school, we were sitting down to take a standardized test of some sort, I forget which, and there was a bit at the beginning where we were required to copy out some kind of academic integrity statement — in retrospect, very silly — but the instructions very firmly said we couldn’t write it “in print”. Hands went up. We asked the proctor if this meant we had to write it in cursive. “Yes, that’s right,” they said. We had to explain that none of us had written so much as a word of cursive for about a decade and didn’t remember how. So the proctor wrote out the statement in cursive on the whiteboard and we all laboriously copied it down, which i’m pretty sure was not the intention. That is, to this day, the only time I have written cursive since the turn of the millennium.