'This building has light, energy, and life!' exclaims Dunbar Senior High School's principal Stephen Jackson. 'I love it, love it, love it!' Jackson is raving about the 280,000-square-foot brick, glass, and steel structure that opened in August in the Truxton Circle section of Northwest Washington, D.C. It will soon accommodate 1,100 students, but for now a few more than 600 are populating its halls. The public high school, designed by Perkins Eastman in association with Moody Nolan, is a four-story facility with an L-shaped plan (one wing for academics, the other'a chunkier block'for sports and arts), partly enclosing an athletic field.