No one familiar with Taylor Branch’s muckraking Atlantic story “The Shame of College Sports,” about the wholesale exploitation of student athletes, will be able to take this documentary about an underdog high school football team completely at face value. But for its subjects, the impoverished and often troubled kids who play football for the Manassas Tigers in north Memphis, a scholarship to play college ball is still the impossible dream. Directors Daniel Lindsay and T.J. Martin focus on the Tigers’ heroically patient and dedicated coach, Bill Courtney, who has to deal with his players’ personal crises as well as their problems on the gridiron. Contrary to the title, the Tigers are defeated early and often, but Courtney really does believe all that stuff about adversity and character, and the film is moving for its modest tale of boys learning to act like men.