I'm a teacher. For years, this was never an issue. Early in my career, kids would occasionally refer to me by my first name, and I always put a stop to that. Recently, the last two years or so, I have been called by my last name often, and not by students who I would have expected. But I hear this a lot among the younger teachers. They have nicknames for the teachers, and usually it's the last name or a shortened version of the last name. I don't know. I'd rather err on the side of respect than the other. And in this case, I think it's a matter of respect.