It was in many ways a typical film-school scene. On a recent hot afternoon, a group of eager young students crowded around a big-time director, asking for advice about backlighting and the best way to establish a scene of anarchy. But the students quizzing David O. Russell, the director of “Three Kings” weren’t enrolled at New York University or Columbia or any other august institution. They were from the Ghetto Film School, an unaccredited training program in the South Bronx that operates in the summer and on weekends during the school year. It gives teenagers a rigorous introduction to filmmaking and, despite the humblest of origins, has built up an enviable roster of Hollywood donors and supporters inside city government. Ben Sisario reports.