WASHINGTON — America's 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 yesterday indicate that fourth-grade pupils 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 pupils' math and reading skills — as demanded by the No Child Left Behind Act — may also be helping in science.

The test found students in grades 8 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 one another.

Education Secretary Margaret Spellings said the results show why the No Child Left Behind Act, 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.

"The answer is more accountability, not less," Spellings said.