Shooting “Afghanistan: The Forgotten War,” a doc for “Now” on PBS, Bill Gentile trekked that country with Marine counterinsurgents for six weeks last year. Meanwhile, summer students in D.C. and elsewhere caught his real-life lessons online via a special “Backpack Journalism Abroad” class. Now American wants it in the catalogue every summer.“I was giving them the whole nine yards about the difficulty of working in severe conditions, the importance of the story I was following, the state of morale of the Marines. All of the things they can’t get in the classroom, I’m sending back to them,” Gentile explains. While shooting his most recent project, “Nurses Needed,” Gentile taught the classroom version of his backpack journalism course, in which individual students film their own docs, with Gentile playing the occasional chaperon. All of Gentile’s classes, which also include photojournalism and foreign correspondence, fill up fast at American. His next distance-learning course will take him to the Galapagos.