SALEM, Mass. — A Massachusetts teenager charged with killing his high school math teacher is challenging the constitutionality of a state law that requires him to be tried as an adult.Philip Chism is charged with raping and killing 24-year-old Danvers High School teacher Colleen Ritzer in October. He was 14 at the time.In a motion filed Monday, defense lawyer Denise Regan argues that the state's "youthful offender" law is unconstitutional.