Grammar and punctuation are usability tools. The writer is allowed to use the whole set of characters, punctuation, and symbols to express himself or herself. There's no law against title my next piece of writing: "Gsfbvsrnbwrkng!#$#jkgfbfart." (And I believe this is, in fact, taught in high schools. It was to me, at least: Our literature textbooks seemed to always include works by e e cummings, for example.) But we have this set of recommendations for writing in the English language that maximizes clarity and comfort when reading. Many high school students don't care about writing. Either they find it boring or don't understand why it's useful or are intimidated by books or something else entirely. So sometimes rules just have to be pounded into them.
And anyone complaining about the quality of public education in the United States, before you get down on teachers, go take a look at what people get paid and what resources are provided to teachers. Especially in poorer parts of the country. We're getting what we pay for.