Philip Chism, who allegedly told police that he “became the teacher” when he slashed the throat of Danvers High educator Colleen E. Ritzer, goes on trial today for the 2013 killing and could become the first juvenile to be convicted of murder since the state granted parole eligibility to teen killers.Jury selection in the horrifying case of a promising young math teacher raped, slashed to death and dumped in the woods begins today in Essex County Superior Court in Salem.