The results found that the reading skills of 12th graders tested in 2005 were significantly worse than those of students in 1992, the first time a comparable test was given, and essentially flat since students took the exam in 2002.

The share of students lacking even basic high school reading skills — meaning they could not, for example, extract data about train fares at different times of day from a brochure — rose to 27 from 20 percent in 1992. The share of those proficient in reading dropped to 35 from 40 percent in 1992. At the same time, the gap between boys and girls grew, with girls’ reading skills more than a year ahead of those of boys.

In math, 23 percent of all 12th graders were proficient, but the exam had been revamped, so the results could not be compared to earlier years. The new test has fewer questions requiring arithmetic, but more using algebra and geometry. Some 39 percent of 12th graders lacked even basic high school math, meaning they could not, for example, figure out the length of a hypotenuse using the Pythagorean theorem.

At the same time, high school graduates in 2005 had studied more than their counterparts in 1990, averaging 360 more hours of classroom instruction during their high school years, the transcript study showed. Their grade point average was a third of a letter grade higher than in 1990, and more students were taking foreign language and other courses aimed at preparing them for college.

Today’s study also compared the transcripts of a smaller group of 17,400 students with their scores on the national assessment, and showed that students taking more challenging classes, like calculus, tend to score higher on the exam than students who take less rigorous courses.