I think if you choose to post on an internet site, writing about your problems, you are probably highly literate. Your children, too, are destined to be highly literate. However, there are huge swathes of people who are unlikely to become this impressive on paper. Yes, partly, it is down to poor teaching, partly ability, often poor home support for literacy. Some houses really don't have books, or papers. Some children really do play computer games for hours every night.
Teachers should not have poor punctuation and grammar. However, you can't correct every mistake because, in some cases, you will stop them from writing fluently and with enthusiasm. It is better (Secondary) for them to write, say, 800 words and make a dozen mistakes rather than feel terrible and write 100 so as to make none. I have seen the results of over zealous correcting by primary colleagues, often with the best intentions and, believe me, it is children (often boys) who just won't write - meaning they cannot really pass any exams.
Yes, correct the teacher. Yes, moan about her / his poor skills. But marking should be specific and targeted to bring about improvement - it is not about the teacher showing off superior SPAG skills.