A mutiny is a rebellion against authority, like when sailors overthrow the captain of a ship or when a class of 8th graders refuses to dissect a frog in biology class.
Mutiny comes from an old verb, mutine, which means "revolt," and a mutiny is still like a revolt. It can be a group of people, like in the famous Mutiny on the Bounty in 1789 when the crew of a British Royal Navy ship kicked the captain out so they could stay in Tahiti. (Totally worth it.) A mutiny doesn't have to be at sea or in the military; it can be to "refuse the order of the person in authority," in other words, every teacher's nightmare.