Cartman is trying to avoid a fight like the main character in "Three O' Clock High". In one scene Cartman anxiously looks at the clock that's about to reach three o'clock, which is a direct reference to the movie.
When Wendy is sitting on ground next to Cartman's bloody body, Mr. Mackey calls her name. She responds by saying "I'm finished." This is like the final scene in "There Will Be Blood", where Daniel Plainview says "I'm finished" after killing Eli Sunday.