He said: "At the school, its sir. In the bar, its <first name>"
No, I'm sorry, in this day and age, I just can't get my head around that!
I always ask for a first name and give them mine. I'd be most put out to be called 'sir', though I hope I'd be gentle in politely reminding them that I have a name. The ONLY time it's remotely appropriate is when meeting a customer as a stranger for the first time i.e. they walk into the school 'can I help you, sir?'
TheOddOne