United States students are improving in reading across the board and in math at the lower age levels, with low-achieving students making the biggest gains. But high school math scores have remained flat since the 1970s–a trend that has many observers worried.

The 2008 scores come from the National Assessment of Educational Progress (NAEP), a federal test considered the benchmark for how students perform across the country. In a report issued April 28, reading and math scores were measured against long-term trends.

Results were particularly noticeable on reading. Reading scores tend to lag behind math scores, but in 2008, students in every age group–9, 13, and 17–made gains. That hasn’t happened since 1975.

In math, scores improved for younger children, but scores for 17-year-olds remained flat.

Education Secretary Arne Duncan said he was pleased but not satisfied with the results.

“We still have a lot more work to do,” Duncan said. “Our focus on raising standards, increasing academic rigor, and improving teacher quality are all steps in the right direction.”

Results were in line with long-term trends, said Darvin Winick, chairman of the National Assessment Governing Board, the bipartisan panel that oversees the test.

Over time, schools have done rather well with elementary school kids, better with middle school kids, and have stalled with high school kids, Winick said.

The biggest gains came from low-achieving students. That is probably not an accident–the federal No Child Left Behind Act (NCLB) and similar state laws have focused on improving the performance of minority and poor children, who struggle the most.

“The big pressure for the last six, eight years in this country has been on bringing the lower-performing students up,” Winick said. “And what this long-term trend says is, generally, that’s what’s happening.”

Tom Loveless, an education expert at the Brookings Institution think tank, said the progress in reading is noteworthy.