Washington ? Elementary school children are getting better in science, but middle and high school students are not, a blow for a nation wary about losing its competitive edge.

Federal test scores released Wednesday indicate that fourth-grade students posted small gains over the past five years, mostly through improvement by the lowest-performing children.

The progress was interpreted by education officials as a sign that greater attention to elementary students’ math and reading skills – as demanded by the No Child Left Behind law – may also be helping in science.

The test found students in grades eight and 12 failed to improve at all since 2000 in their knowledge of earth, physical and life sciences. The high school seniors actually did worse in science when compared with scores of a decade ago. Almost half of the 12th-graders taking the test in 2005 fell short of showing basic science skills.

The science scores are from the 2005 National Assessment of Educational Progress, a federal test given periodically on a range of topics. It is considered the best yardstick of how U.S. students perform over time and of how states stack up against each other.

Education Secretary Margaret Spellings said the results show why the No Child Left Behind law, which focuses on early and middle grades, should be expanded in high schools. The law requires testing in math and reading, with penalties for many schools that fail to improve.

State science testing under the law will begin in 2007-08, although schools will not face consequences for their performance – something President Bush wants Congress to change.

Science skills have become critical in a huge range of blue-collar and white-collar jobs, and they form the foundation of engineering, technology, medicine and other leading fields.

Only 30 percent of 12th-graders took biology, chemistry and physics during their high school years.

On the test, most students in each grade could not handle challenging subject matter.

At that skill level, known as proficient, a fourth-grade student can explain what can be learned from fossils. An eighth-grader can identify the location of a cell’s genetic material, and a 12th-grader can design an experiment to compare various heating times.

Only 29 percent of students in grades four and eight scored proficient or better, and only 18 percent of 12th-graders did.