I agree with most of the explanations above.
However, let �s look at it from the side of the teacher and students.
Are the students enjoying the lessons?
Are they learning anything useful?
Is the teacher doing his/her best?
Are the lessons well prepared?
If most questions above are answered with yes, I don �t think this little grammar item is worth making a fuss about. How would you feel if every single parent was poring over everything you do in class and then request a meeting to discuss some minor grammar or vocabulary point?
I agree that teachers should be perfect and know everything inside out, but isn �t there more to learning a language and becoming fluent than just reciting grammar rules?
Nobody �s perfect.
BTW, many posts on this forum contain mistakes. How can that be if we are all English teachers?