The teachers in my building generally refer to each other (and even the principal) by last name only. The kids call us by last name. Nobody seems to care. I see it as almost a term of endearment. I came out of the military, and nobody ever called me "sergeant" unless they didn't know my name.
I don't think the kids mean any disrespect. If you want to be called Mr., correct them. If another teacher wants to be called by their last name only, what's the point of correcting students who do that?