Famous last words. That's what those are. There are a few places you never mention how calm it is; the hospital, a restaurant and a school. See, the universe has this wonderful joke that anytime you mention what a nice, easy time it seems like you're going to have, you'll have the opposite. Why? Who knows, really. I suppose it's Murphy's Law to the extreme. Once you utter any form of phrase that insinuates it's totally calm today, everything that can go wrong, will go wrong.
It's not just when you're talking to other teachers, either. No. You can't mention it to the students either. I learned my first year that saying something like "Wow, you guys are being really nice and quiet today!" is a sure fire way to make a fight to break out, a window to get broken, someone to break up with their significant other and someone to projectile vomit everywhere all in about fifteen minutes. Is it really that extreme? No. However, when you're in the middle of it and you're in charge, it seems about twice that bad.
It comes down to expectations. You can't go in expecting the worst and being delighted with normal. Mentioning to your students that they are being so good as if you are surprised tells them, even on some deep down, subconscious level, that you expect less of them. So they'll meet your expectations and then some. Instead of being delightful surprised that your students have behaved like decent human beings, try pointing out specific behaviors to specific students what you like and would really enjoy seeing again. Walk around with a stamp. I know it sounds corny, but I did this yesterday during group work. Everyone who was doing what they were supposed to be doing received a stamp that said "Awesome!" with a star next to it. It's totally corny, but they loved it. The stamp craving spread like wildfire. Soon, everyone was doing exactly what I wanted asking "Do I get my awesome yet?!" By the end of class, everyone had their awesome and quickly, and quietly, finished their work without a problem. You see, you could earn multiple awesomes and now it was a race to see who could get the most before the bell rang.
And these are high school students. The only thing that would have worked better is walked around with a bucket of candy. However, the candy has a few drawbacks. First, I don't want some other teacher loading my kids up on sugar and then sending them to me to deal with, no sir, so I won't do that to someone else. Secondly, unfortunately, not everyone can eat all of the candy, so some kids will be left out. Finally, candy runs out. I can get more ink onto that stamp and I don't have to keep buying giant bags of M&Ms to get my kids to work.
You'll learn that in teaching and in life, you get what you expect. Of course, just wanting perfection won't always achieve that outcome for you. You have to do some leg work before you can expect everything to run smoothly. Having good, established classroom expectations from the first day of school is a start. Being firm and fair with those expectations is also a necessity. The problem really is when you expect only bad things. It becomes a self fulfilling prophecy and one that is hard to escape.