American students are less proficient in their nation’s history than in any other subject, according to results of a nationwide test released on Tuesday, with most fourth graders unable to say why Abraham Lincoln was an important figure and few high school seniors able to identify China as the North Korean ally that fought American troops during the Korean War.

Over all, 20 percent of fourth graders, 17 percent of eighth graders and 12 percent of high school seniors demonstrated proficiency on the exam, the National Assessment of Educational Progress. Federal officials said they were encouraged by a slight increase in eighth-grade scores since the last history test, in 2006. But even those gains offered little to celebrate because, for example, fewer than a third of eighth graders could answer even a “seemingly easy question” asking them to identify an important advantage American forces had over the British during the Revolution, the government’s statement on the results said.

Diane Ravitch, an education historian who was invited by the national assessment’s governing board to review the results, said she was particularly disturbed by the fact that only 2 percent of 12th graders correctly answered a question concerning Brown v. Board of Education, which she called “very likely the most important decision” of the United States Supreme Court in the past seven decades.

Students were given an excerpt including the passage, “We conclude that in the field of public education, separate but equal has no place, separate educational facilities are inherently unequal,” and were asked what social problem the 1954 ruling was supposed to correct.