Most of the parents try to change their kids' behaviour when they are displaying turbulent behaviour. For example, when a kid is angry and is throwing things around, trying to change him that very moment may fail.
In fact, when a kid is overwhelmed with certain emotions, your preaching might not work.Instead of that, sit with your child when things are normal and train him to deal with anger or other emotions. It is better to be prepared before a storm as you can't do anything when the storm is already there.
Also, trying to suppress a child's emotions may not be a constructive way. Instead, anger management proves better. Explaining the kid to channelise anger or handling it with grace could help him even when he grows up into an adult.
Also, knowing the reason why a kid is angry helps you deal the issue better. Is the kid craving for attention? Does he feel ignored? What caused him frustration? Is he suffering any medical condition? Was he treated badly by someone in the school?
Identify the reason first before you teach him how to handle such emotions. Without the ground work, things can't be solved at all.