An Orinda girl convicted of stabbing to death a classmate in a killing that stunned the quiet community where they lived was sentenced to the California Youth Authority for up to nine years. Bernadette Protti, 16, was convicted last month of the second-degree slaying of Kirsten Costas, 15, in June. In a tape-recorded confession to the FBI that was played during Protti's non-jury trial, the Miramonte High School student said the pretty and popular cheerleader symbolized the social and academic success that seemed to elude her, and that she stabbed Costas in a jealous rage. Contra Costa Juvenile Court Judge Edward Merrill said the length of time Protti spends with the CYA will be determined by the authority, and she cannot be kept there past the age of 25. She will remain there for at least three years.