When I was in the infant and Junior school 1953 to 1959 we had to stand when any adult entered the class room. I don't think it carried over into Grammar School.Would that be Sarf London ?
I'm not going to denigrate all teachers, but when I see them in schools (and my niece is a Head Teacher), it doesn't seem to be in their remit to pull up children on their method of speech.. Even a simple sentence "it ain't right Miss" should be corrected at the point its spoken. Sadly many teachers are not exactly articulate themselves, so children haven't a chance. Accent is not a problem but enunciation is.
In my school days - 1953 to 1965, yes it was a different time, but our teachers looked, dressed and behaved like our parents, so if we played up we were disciplined, if we spoke in slang we were corrected, but teachers today look, and dress like our older siblings so the lines become blurred.
As for 'guys', I rebel. Just let anyone dare to call me 'guy' in a shop/restaurant or bar, and I call them out on it. Even when I'm handed a bottle of drink instead of a glass I say "do I look like the sort of person who would drink out of a bottle ? " If we all give in who knows where it will end.
I remember one Christmas at my gran's my uncle came in with one of his friends I hadn't seen before and I stood up.
I use punctuation and capitals when texting BUT have been guilty of using ,"guys"quite a few times.