I'm not really sure there's a rule here. In French we have the plural form when we want to be formal, but then being formal is not as mandatory as it was 20 years ago.
For example, when I was a kid, I would always call my teacher Mrs Bonnet or Mr Joubert, and as far as I remember I'd use the plural form with them. Everybody did. These days, kids call me by my first name and use the informal singular form with me. They do because I asked them to. Before this year, I was teaching in a countryside school where kids just couldn't manage to be formal with us, and so it made sense for me to expect the same from my pupils here, especially as it makes me feel very old when they call me Mrs Whatevermynameis. But a couple of parents expressed surprise and even a bit of shock when they realised that kids didn't use the plural form with me.
Have to say I was a bit taken aback at first last year, when I spent a month teaching in Manchester and my tutoring teacher asked the kids to call me Mrs Whatevermynameis instead of my first name.
However, among teachers, whether it's in the UK or in France, we call each other by our first name. In French there's even a tacit rule saying that we have to keep using the informal singular form among teachers, regardless of the age difference. Same when you're part of the same political party or the same union.
I think that in it really depends on the individual person, though. My friend Maud has the hardest time in the world being informal, whereas I have the hardest time in the world being formal. For example, she still uses the formal plural when she talks to my parents, whereas I'm completely informal with hers.
Sometimes it can be an issue, too. Last year, I had a teaching practice at an infant school, and in infant schools there's the teacher, and another person (not a teacher) who helps out with the kids and the activities. The teacher I was replacing had a very complicated relationship with the woman working with her in the classroom, which had all started because she'd refused to go informal with her as they didn't have the same status and she wanted to maintain some distance. I couldn't see myself be formal with a person I worked with for 6 to 7 hours a day, and so I was informal with her and we had a fantastic working relationship. Other times, though, it can be the opposite, and maybe someday I'll end up meeting people who will be truly shocked if I try to be informal with them. Oh well.
Anyway, sorry for rambling on.
Kaethel
