While author Elin Hilderbrand has officially retired from publishing a “Nantucket” summer novel each year, she recently embarked upon an adventure to write a boarding school book with her daughter, Shelby Cunningham. Cunningham and one of her older brothers opted to attend boarding school rather than graduate from Nantucket High School, and Hilderbrand has made it no secret that she’s always been fascinated by the the settings of these exclusive schools. So while she didn’t attend one herself, she collaborated with Cunningham to create a fictional school set in Massachusetts called Tiffin Academy in their latest novel. Her daughter graduated from St. George’s School in Rhode Island. “The Academy” follows a colorful cast…