Kids often misbehave, or simply behave differently from what you expect from them. They do so for different reasons. As a teacher, it is vital you are aware of these reasons, as this can make you more competent in tackling the misbehaviour.
There are many things you can do. The website at the bottom provides useful information on why children might behave the way they do, and also gives an accurate description on how to talk to a child during a ‘serious talk’.
I strongly recommend reading the information, as it will strengthen your ability to handle certain problematic situations, and will make you understand misbehaviour better.