I worked with a French woman a few years who liberally peppered her conversation with mine de rien (mean duh re-en). She seemed to use it in ways that shifted, so I was never 100% sure of what it meant and how to use it myself. There was good cause for my perplexity. This expression has a variety of English approximations, but it seems to be closest to “as if nothing had happened” or “as if butter wouldn’t melt in his/her mouth.” It seems to be a synonym for the French expression “comme si de rien n’était,” with which I am more familiar. You can imagine why this expression would come up frequently in a teacher’s world. There’s many a wide-eyed innocent look when it’s time for homework to be submitted.