New teachers are often told "not to smile until Christmas". The idea is that once you lose a class it is very difficult to get them back, so it is imperative that you make sure that you know your students and how to deal with them, before you let your guard down.
You need to know how to control them better than they know how to control you. The way you control any class depends on what size class you have and also on the age of the children, but here are some tips on how to keep your class together.
Getting your lesson well planned out and all your work ready is essential to keeping your class under control. If you spend time looking for books or fumbling through sheets of paper your children will get restless. If you have everything planned and ready the children will see a well-organized teacher that knows what they are doing.
Don't give your children time to stop and think about talking or causing trouble. You can give them a break but keep it controlled. With time to sit around the children will no doubt find something else to do and it probably won't be something you like.
Keep lessons interesting
Just like we all hate boring staff meetings, children hate boring lessons. Make sure all the activities are pitched to the children and made as interesting as possible.
If they like what they are doing, they won't want to ruin it. It may take a lot of planning time outside the class but if you are having control issues, it will save you the headache inside the class.
Remember to differentiate work appropriately. Work set too high for struggling children will see them give up early and work set too low for your higher ability will see them finish early. The busier your students are the better. Make sure you always have something else ready for them to do.
Be fun and engaging
Body language and tone of voice are your best weapons when controlling a class. If your voice is dull it doesn't matter how good your lesson is, your students will be disinterested. Be energetic and entertaining.
Show your students you care about them- Most children might not show it, but they actually want to learn. If you don't show that you have time for them, then they won't respect you.
Similarly children that play up usually just need a teacher to support them. You will get a lot more out of students when you speak to them like grown-ups and ask them why they are playing up.
Even in Kindergarten I discuss matters like the children are adults. If you are too authoritative you will create combative relationships with strong willed pupils and no one wins from there. In the long run, shouting children down never works. The children will just become accustomed and then you will have no other cards left to play.
Rewarding good behaviour
Praising impressive work really helps children feel a sense of accomplishment. Sometimes it is hard to find things to praise about the more disruptive children but when you do, make them feel like they are worth something.
Don't praise poor work but grab any opportunity to give helpful feedback and make the students want to do their best. Be careful not to over-praise as this devalues your feedback.
Probably the most vital piece of advice you can get. Don't let good students get away with bad behaviour just because normally they are good or that you know they won't get out of hand.
The other children in the room will take note of this too. It gives them a reason to misbehave and a reason to think you are unfair. If you have a difficult class, don't allow them get away with little things.
Controlling a snowflake is a lot easier than controlling an avalanche. If you are finding one child particularly difficult, focus on the rest of the group first until that child is misbehaving alone.
Children often misbehave for attention, so if they lose the respect of the other children, then more often or not, they become less disruptive.