"Tardy Theresa, I noticed that you've been coming late to class a lot lately. Is something preventing you from coming to class on time?"
"Nope. I'm just late," she chirped.
"Uh, don't be late?" I said, not quite sure how to respond to such cheerful admission of fecklessness.
Well. There was a teaching moment well-seized. I'm sure that resolved the issue completely.