My mature students called me by my forename as I did them. There's a book about adult ed called 'Teaching on Equal Terms'. One can't do that if there's some social distinction. Amusingly, when I taught O level English to RAF students, mostly NCOs, they had to be dissuaded from calling me ma'am, as if I were an officer. Informality finally triumphed. When I was teaching ESOL, my Iranian students couldn't quite stomach calling me by my forename and compromised with 'Mrs A'. Teaching teenagers, of course, was a very different matter. If you don't maintain a certain distance, there's going to be a breakdown in discipline.