If you want a behavior changed, there is one thing you always need to do. Be specific. Sometimes teachers fall into the trap of trying to be "nice" and not call students out. While you don't want to make a huge scene out of discipline, making blanket statements doesn't do anything. I used to do this constantly. Three or four students would be misbehaving, so I'd say "Some people aren't being very respectful in here."
Here's what happened in that classroom. Some students who were actually being good are going to think you're talking about them. They'll fret and worry over what they've done wrong and now their focus is completely gone. The kids who were actually misbehaving may have no idea that you are actually talking about them. Believe it or not, not all students understand what they are doing is wrong, or more importantly why it is wrong. You also have not told them what behavior you expect instead. Maybe one of the kids was talking and he actually understood you meant him, so now he's writing notes instead. Now you get to make your non-specific correction again and he'll just move onto a new game.
I've discussed dealing with disrespect in another post. The same rules still apply. You don't want to do a major call out of a student because you're challenging their pride in the classroom and they will defend it. Instead, use something called "proximity". Proximity as a form of discipline works well. Some students will stop whatever activity it is just because you're standing near their desk. I usually get down on their level by kneeling or bending down and ask them to quietly stop the behavior. You need to tell them what you want them to stop and give them a behavior you expect instead. "Johnny, you need to stop talking. You need to be focusing on the lesson right now." One major thing from that correction to note is that I didn't say "I want you to" or "I need you to". Teaching is not about you. It is about the student. They do not care what you want or need. They care what they need to do to pass.
If a behavior is persistent, I take the student in the hallway and ask them "Why is talking during class wrong?" As frustrated as it may make you, some students do not honestly understand why that is a rule. It's just a rule that someone told them in kindergarten or first grade and no one ever explained why you're supposed to do it. "Johnny, talking when I'm talking tells me that you don't respect my time. It tells me you think the lesson is worthless. More importantly, when you talk, you miss directions. Even if you didn't miss the directions, someone else might because you are distracting them."
Along with specific corrections, you need to make sure you're giving specific praise. "You did so well quietly taking out your books!" is much more likely to get a repeat of that behavior than "Good job today, guys!" Good job at what? We did a lot of stuff today and not all of it was good. There's nothing wrong with being positive, but you need to be positive by pointing out the specific behaviors you want to happen again. Again, avoid phrases like "I like the way you sat down quietly." They still don't care what you like and you've made the praise more about you than them. They did well so you want to make sure you're praising them.
Being specific is a tool that helps in just about everything. Problems in your relationship? Be specific with your boyfriend! Is your own child doing a behavior your don't like? Tell them what they need to stop and why they need to stop it. Things that are often obvious to adults aren't always as clear to children. It helps to remember that children and adolescents are not miniature adults and they don't have the exact same experiences and knowledge as you to understand what it is they need to do.