I too was a convent girl, who was caught not wearing a hat. Members of the public used to phone the school to report us and we would get a rollicking from the nuns, as "ladies should always wear a hat outside." Also, we used to roll up our skirts and the nuns would pull them back down as we passed, saying "It is so common to show your legs". !
We very very poor when I was young, but I had a school friend, whose parents were very well off. I was invited to lunch in their large house and was so out of my depth. I have always remembered the look on the mother's face, when she put a plate of watercress soup in front of me and I said I didn't know what it was. There were other comments made. In hindsight, I realised that she thought I was too common and not a suitable friend for her daughter.
The irony was, that her husband told me that when they were first married, they were so poor, that they lived in a "single end" in Glasgow, with only a bed and a table and two chairs.
A "a single end" was one room with a sink, a bed recess and an outside toilet on the stairs in a tenement".
Not using a knife and fork properly and eating and talking with your mouth full, are the ones that get me.
I think that if I do the posh quiz, I will find that I am still common. I remember Soop joking at the Edinburgh bash that she expected to meet a very posh lady in Marydoll, and I had shattered her illusions!
As my friend always says: " You can take the girl out of Glasgow, but you can never take Glasgow out of the girl!"