Do Sweat the Small Stuff
In those first few minutes, hours and days in the classroom, you are essentially creating a world. And you want a world in which students do things that will keep them or put them on a path to a life replete with meaningful opportunities. Behaviors or actions that will detract from that world should be nipped in the bud. If you only "sweat" major misbehaviors, students will get the sense that minor misbehaviors are OK. If, on the other hand, you lovingly confront even the smallest misbehaviors, then it will be clear to students that, inside the four walls of your classroom, things that detract from what you're trying to achieve – even in small ways – just don’t fly.