Boarding school novels, from John Knowles’s execrable “A Separate Peace” to Curtis Sittenfeld’s excellent “Prep,” have long been a staple of American popular literature, even though almost no one in America goes to boarding school. Readers simply have a persistent interest in the ribald and bittersweet experiences of young people away from home for the first time. But while Louis Auchincloss’s affected but engaging new novel “The Headmaster’s Dilemma” (Houghton Mifflin, 177 pages, $25) takes place at a boarding school, the students are a virtual afterthought — it’s the administrators who get all the attention.