So let's say you're teaching a topic close to your heart, something that means a lot to you, and has for a long time.
(Brief interruption: If you never have that feeling, getting up in the dark every morning to go stand in front of young people for very little money may not be the job for you.)
So, as I was saying, there you are expounding on, say, the beauty of the quadratic equation, and you notice either Jethro or Selmolina in the back of the room doing something that, while not harmful or dangerous, s/he knows full well s/he should not be doing. The offender may even be aware that this act of rebellion is within your line of vision.
Meanwhile, though, the rest of class is completely entranced by the quadratic equation, especially the ax2 part. Students are taking notes. Their eyes have that priceless "this-is-why-we-go-to-school" look. But you're furious with that punk in the back row.
What to do?
Unless the student is disrupting the flow or continuity of the class, you just look the other way. The strategy could be the same even if it involves two kids, maybe even three.
Let them have their little world into itself. The consequences will come down on their little self-centered heads soon enough. If you single them out to call attention to the fact that they're doing something wrong when they already know that, you're the one disrupting your class, not them.
Now everyone is watching an exciting showdown between a frustrated adult and an attention-seeking adolescent. Who will win? Will Jethro or Selmolina get written up? Will the teacher become overwrought and let slip a profanity? Tension grows in the room, especially for students suffering from any degree of anxiety issues. All fascination for the quadratic equation -- despite the charms of ax2 -- is long gone. What are the odds of your winning them back?
So do you then talk privately with the offender(s) after class, have a little heart-to-heart, remind the little knucklehead who's boss, threaten a call home to the parents, that sort of thing? I wouldn't even do that, not for a while, anyway. The student was seeking attention, but didn't get it. The other students continued to learn. Life went on. That wasn't any fun. Maybe tomorrow she'll try listening to you for a change. And if that happens, you can dish out some moderate praise for her decision to do what students are supposed to do.
In short, new teachers, it may take you a while to figure out which fights are worth engaging in. But whatever you do, don't jump into all of them. Adolescents hate to lose, especially in front of their kind. And when they do lose, their relationship with you is likely to be prickly for weeks to come.