There is some truth to teachers not marking errors because they don't know grammar. For instance, I once had another English teacher interrupt my class and wanting me to tell what was correct about a grammar problem that had come up in her class.
And then some teachers are just too lazy to go to the trouble of marking errors. (I've actually heard some of them admit this.) Some don't do so because they fear parents complaining to the principal for giving their children low grades for "unimportant things" like grammar. (I often heard the complaint about me as being "too hard".)
Parents like the ones you described get on my last nerve. Do they really think they’re doing their kids any kind of favor by trying to get teachers to be more lenient or are they just lazy? Maybe they should remember they won’t always be around to help their kids write resumes and reports and that grammar does count in the corporate world.
My dad had to hire some low paying help once and he told stories at the dinner table about the resumes. It was rather sad but one was pretty funny and I’ve never forgotten it. Under the line that sames “Sex”, some wanna-be employee wrote “as often as possible”. LOL
That is just too darn bad about you being “too hard”! I’d bet those kids who were in your classes now mentally thank you