The Plastics split up. Regina George (Rachel McAdams) survives the bus accident. Cady (Lindsay Lohan) joins the matheletes, answers the final question correctly, and the mathletes win the state championship. At the Spring Fling, Cady apologizes for writing the "Burn Book" but never exposes that it was actually Regina who wrote it. Cady is voted queen, but breaks her crown and gives pieces to her competitors and throws the rest of it into the crowd, keeping a bit for herself. She says everyone is equal. Flash-forward to the new school year: all kinds of people hanging out and talking together. Regina walks by and sees Cady. They smile at each other. Cady sees freshmen Plastics and imagines them getting hit by a bus.
Thanks, MClar488 & Nutmeg1435!