Traditionally, this has been the basic advice for new teachers - all teachers for that matter. In an effort to "manage" a class the teacher is told not to smile or be warm to their students. Many years ago, I tried. I really did. I desperately wanted to be a good teacher - but instead, I was miserable and so were my students.
I believe in the deepest place that being firm, fair and consistent is not mutually exclusive to being warm, kind and approachable. I run a tight ship - I do. My students know my expectations but more importantly, I know theirs. They expect to be safe, loved and challenged. So - instead of "Don't Smile to Christmas" I live by the saying that I had applied to my classroom door this year....
Learning is messy, noisy and chaotic. We laugh and create and even sometimes cry. You're welcome to dropy by anytime - but put on your happy face!
