A mutineer is someone who rebels against authority. If a group of kids refuses to go back inside the school when recess is over, you can call them mutineers.
Mutiny is the act of revolt or opposition against an authority like the captain of a ship or the commander of an army. Anyone who acts to carry out a mutiny is a mutineer. Both words stem from the old verb mutine, "revolt," and its root word meaning "to push away." Formally, a mutineer is someone who's guilty of a crime, but you can use it informally for any type of rebel.