(continued) Likewise, I've heard and read Quakers using "thee is" instead of "thou art", and so forth.

As for the original question, I agree with Mooh that the teacher should not have used "yous" in her conversation during her meeting with her pupil's parents, precisely because it leaves parents with the impression that Mooh describes: that the teacher may be derelict in her duty to teach her students the proper use of grammar and, perhaps, in other teaching responsibilities as well. As a teacher, one is a walking advertisement for one's school and its programs, so one needs to be "on duty", in a sense, constantly, always watchful of one's behavior, particularly in the company of parents who are the customers for the business of education.

But, then, I too have left others with the impression that I am too judgmental (but never "to judgemental"! *G*).