What is the science fiction novel that about a group of kids who grow up in a world that is run by a videogame?

these kids start out as just these kids in this world that is completely run by a video game....like the game is the judicial system and government....and its hard bcuz everyone is just trying to make do with their simple little characters....then one kid decides to make a different character and its a girl and then he convinces his other friends to go fight a dragon on the game cuz he figured out how to take it down....then they beat the dragon and they get rich and they move on to do other quests and then the government ppl who are corrupt are tryin to keep them from giving ppl hope so they use this super character to try and kill the kids characters....then the kids find out that the game is so old that it has created another version of itself and so in turn the game is basically fighting itself and the kids find out how to ultimately actually beat the game which everyone thought was impossible