Always Remain Calm and Focus like a laser on the misbehavior. Kids are smart, they only do bad things when they want to get caught or they don't want to get caught.
The first kid who wants to get caught is simply looking for authority and order in his/her life. Pay close attention to the misbehavior, give them a reasonable punishment and make sure they do the punishment. Sometimes these kids will grandstand and try to argue/challenge you in front of the group. They figure that whatever punishment you will give them is outweighed by the respect they get by standing up to you. That is not acceptable. Take them away from their audience and speak with them privately, like ouside the room where the other kids can't hear. You will find they are much more cooperative then.
The second kind of kid does not want to get caught. For that kid you need to increase the risk for them by letting them know that you will STOP EVERYTHING and address any behavior that crosses your line.
Draw a mental line in your head of what behaviors cross the line. Then you have to be consistent and stop everything and focus on the bad behavior and punish or lightly shame the kid for doing that behavior. If you find kids are still pushing up to the line, you need to be more unpredictable and start punishing kids for coming near the line. The goal here is to let them know that you are fully focused on discipline, and that since they will never know exactly where the line is, that
they won't come near it.
One trap you can fall into is to to clearly state the rules to them so that if they ever get punished they can play lawyer and say that they did not technically deserve it. Never get in this conversation, especially in front of the others.
The absolute worst thing you can do is ignore bad behavior until it makes you angry then explode, or continue to ignore it. This is the least efficient method.