Defying her high school's couples-only tradition, a 17-year-old girl attended the Edison High School prom without a date over the weekend after a dispute that divided the student body. Dressed in a green gown and long black gloves, Shawnda Westly of Huntington Beach rode to the dance in a white limousine provided by a sympathetic radio station. Joined by a male friend who also came dateless, Westly danced in front of the Elks Lodge stage while disapproving couples grumbled in the background. "It bothers me having her here," said Brian Martinez, a junior. "It's a bad way to get attention." But support came from Miss Westly's father, a medical electronics salesman, who posted a note on the kitchen cabinet that read: "Don't forget, you're right."