Face the Music, an ensemble of teenagers based at the Kaufman Center, has 74 musicians — up from 8 at its inception. . . . when the conductor, Jenny Undercofler, using a pencil as a baton, leads her Face the Music ensemble through Steve Martland’s rhythmically pointed, texturally jagged “Horses of Instruction,” you realize that this is not the sort of student orchestra you may remember. It is instead an alternative-universe high school band: a new-music factory that focuses on works written in the last 50 years, with an emphasis on the new alt-classical style.