As both an adjunct at a community college and a part-time teacher at a middle school, I have experienced a vast range of students and instructors. When I first started teaching grad school, it was almost a competition amongst the grad students to see how many students they could scare into dropping their class the first day. In middle school, of course, students don't have that option. Instead, 'don't smile until December' is about setting the tone of your class, of making sure that students understand that they are going to have to do the work if the want a good grade.
But I can't help think that this idea supports several statements I strongly disagree with. It says "learning is serious business", "this is work, being a student is your job", and worst of all, "learning isn't fun".
But the truth is, learning should be fun. Learning is exploring new ideas, new places, new ways of thinking. School is where children learn how to become adults, how to communicate with one another, how to make friends and socialize, and how (hopefully) to one day make the world a better place. And if your teacher isn't smiling, well, who can blame kids (or adults!) for not liking school?
Don't get me wrong, learning is serious business. And it is a student's job to do the best they can at school. But any employer can tell you that they would gladly take one employee who loves their job over a half dozen who don't.
It takes a lot of energy to smile day in and day out. Students can be exasperating. It takes a lot of work to make conjugating verbs into something fun and exciting (believe me... it takes a lot of work), but the pay off can be worth it.
I have noticed that some of my students (college and middle school), are taken aback by my teaching style. They aren't sure how to react to the teacher jumping up and down in front of the room shouting, "aren't Latin nouns cool!" or "Maps are great!". But eventually they loosen up, and many are surprised to find that learning can actually be fun.
Legall-ly stuff: This are my opinions, and do not necessarily reflect the opinions of my employers.