Hilary Duff‘s eldest child, Luca, has started high school. Duff, 38, posted a photo of her and ex-husband Mike Comrie‘s 14-year-old son on his first day of ninth grade via her Instagram Story on Thursday, September 3. She also offered a glimpse at her daughters Banks, 7, and Mae, 5, as they prepared to return to school. (Duff and husband Matthew Koma share Banks, Mae and daughter Townes, 2.) Duff and Comrie, 45, welcomed their son in March 2012, less than two years after they exchanged vows. After nearly six years of marriage, the former couple finalized their divorce in 20...