The U.S. Department of Education has joined the District’s investigation into allegations that some recent big gains on standardized test scores might have been the result of cheating by teachers or principals, a D.C. official said Thursday, reports the Washington Post. Roger Burke, a spokesman for D.C. Inspector General Charles J. Willoughby, said investigators from the Education Department’s Office of Inspector General have been active in the probe, which was requested in March by Schools Chancellor Kaya Henderson…