Ditson Elementary School principal William Downing will retire this summer after 34 years at the helm of the 700-student school. Downing, 68, has worked in the Billerica public school system for 46 years as a teacher and administrator. Eileen Roake, the current principal at the Vining Elementary School, will succeed Downing. Roake had served as Downing’s assistant principal for 10 years and knows Ditson well. Victoria Hatem, Downing’s current assistant principal, will replace Roake at the Vining.