A proud socialist, director Ken Loach has spent his career studying working-class folk with the same discomforting intensity that Woody Allen employs whenever he trains his camera on Scarlett Johansson. While he’s set class-obsessed tales in Ireland, England, and the United States, Loach’s heart is in Scotland, and 2002’s Sweet Sixteen is a stirring portrait of life on Glasgow’s bottom rungs. As Liam, Martin Compston plays a young man ruled by his mom’s domineering, drug-dealing boyfriend. Mom would help if only she weren’t in prison; his sister would help if only she weren’t so desperate to make her own escape from the slums. If Scotland features a color besides gray, Loach isn’t letting on, and the burrs are so thick it may help to have a Glaswegian handy to translate. But Liam’s story is compelling even while it’s despairing, and Loach successfully blocks his usual instinct to turn his stories into lectures. SWEET SIXTEEN SHOWS AT 7 P.M. AT THE LIBRARY OF CONGRESS’ MARY PICKFORD THEATER, 101 INDEPENDENCE AVE. SE. FREE. (202) 707-5677.