When was it announced the Tennant and Piper would be returning to the show ?
After the appearance of Sarah Jane Smith in the 2006 episode " School Reunion " , it was announced that Elisabeth Sladen would reprise the role in a new series entitled " The Sarah Jane Adventures " , the first episode of which aired on BBC One on 1 January 2007 , followed by its debut as a weekly series in September 2007 .