Well, maybe it's just me, but I'd be embarrassed to make such basic grammar errors in any situation, whether I was 'advised not to worry' or not.

Same here, but I've observed that this just isn't the case for a lot of other students. I took a philosophy class last year, and I quickly learned that some people just don't care.

Now this I agree with. I had the unfortunate experience of an entry-level English class that practiced peer review / cooperative editing; it can be effectively summed up as this repeated fifteen or so times over the course of a semester.

I strongly agree, and I've had a few similar experiences (though not as bad the one as you described).