I don't equate teaching Standard English and expressed concern for it as mocking.
It depends how that expressed concern is expressed, doesn't it?
I rarely encounter it in native English speakers.
It's just
a Google away:
15,500,000 English pages for
ain't
Your personal experiences of ESL students who habitually say ain't and teachers who intransigently insist on whom at all times are totally foreign to mine.
But we can still discuss those students who do use, or do ask questions about,
ain't, can't we?
But regarding professional teachers in the classroom, I just don't see it.
Happens all over langauge fora.
I think it is concern for the well-being of students, in any field, be it math, science, or geography.
Sounds like "teacher as saint", to me.
Any race of people should be proficient in all these areas as well as others.
You expect whole races to be proficient in such things?!
Mocking ebonics is racist, but teaching standard English is not:
Unless, of course, one uses "scare tactics" - such as promises of eternal damnation, economic failure and inviting ridicule - to teach Standard english. It happens.