"The last war our world has known is called the "101-year war". Fire lord Sozin wanted to rule the world, with Fire as the superior element. He started the war 12 years after the death of Avatar Roku. Roku was an Avatar from the Firenation, so the new Avatar would be born in the Airnomads. That's why Fire lord Sozin started the war by attacking the Airnomads. All of them were killed, except for the Avatar.
The new Avatar was Avatar Aang. He was stuck in an iceberg for a hundred years. When he got out, he found out about the terrible war. He had only till the end of summer (it was winter when he got out of the iceberg) to defeat Fire Lord Ozai conquering the world. Sozin's comet would arrive in the end of summer, giving great power to all firebenders.
Avatar Aang succeeded. He now had the difficult task of restoring balance to the world. The world would never be the same again, because the airnation was destroyed forever.
Avatar Aang believed it was best if the people of the four nations would live together. If the people learned to except each other, just like him and all his friends from the four nations, the world would be in peace. That is why Avatar Aang decided to create a school were people of the three nations could learn of the other two nations. They would learn about the other cultures, with people of the other nations. Benders were also taught how to bend their element, and non-benders learn different fighting techniques (girls can choose how to fight Avatar Kyoshi-style for example).
The school was called Eirene. (Eirene meaning peace in an old language) It wasn't a big success at first, but more and more parents send their children to Eirene. Nowadays it's a popular school, where almost everyone goes to learn."
"History Of Our World 5" by Koko of Kyoshi Island