I have seen too many teachers arguing with children, raising their voices, even screaming at a child who is out of control. Of course this is completely useless, and counterproductive. You are the adult, and it is your role to model mature problem-solving, not resorting to the same inappropriate behavior the child is using. Even if a child is shouting obscenities, or saying hurtful things to you, it is essential NOT to react emotionally.
It can help to remember that this outburst is not about you! It is about a child not having the coping skills to handle her challenges. Do not take it personally! Think of yourself like a firefighter. There's no point in getting angry at the fire - you need to put it out as quickly as you can to minimize damage. You can't do this well if you are as agitated as the child.
So, how do you learn to stay calm and not react? The same way you learn any new skill - lots of practice! Work on being able to notice your body's react to children's inappropriate behavior. Do you tighten your shoulders? Furrow your brow? Get a lump in your stomach? As you learn to pay attention to your body's signals, you can realize more quickly that you are agitated. The more quickly you take care of your anger or fear, the easier it is to stay calm. This is true for children, too. If you can redirect them when you first notice their agitation, you can often prevent an outburst.
When you begin to notice your body's agitation signals, the first step is to breathe! Give yourself a moment or two without doing anything except following your breath. Smile, and try to say to yourself, "This isn't about me, this is a child who needs my help." It helps a great deal to practice physically relaxing your body. Here's a one minute relaxation video to help you practice. If you download it to your phone or ipod, you can practice a few times throughout the day.
Let us know what strategies you've found to help yourself stay calm when children push your buttons!