The Nation's Report Card gave American schools an "A" Tuesday for helping students master the basics in reading, writing, math and science but flunked them for not teaching practical use of those skills.

The Nation's Report Card, sponsored by the National Assessment of Educational Progress, said schools have reversed negative performance trends, and the review pointed to improvements in students' ability to do simple computation, comprehend simple text and learn simple everyday science facts.However, Archie Lapointe, executive director of the Report Card, said "Large proportions of American students do not appear to be adequately prepared for college work, career mobility, and thoughtful citizenship."

The report is based on data from more than 1 million students aged 9, 13, and 17, and assessed in seven subject areas since 1969. The key findings were:

-About 61 percent of 17-year-olds lack the reading ability necessary to find, understand, and explain relatively complicated information. But students at all three ages were reading significantly better in 1984 than in 1971.

-More than one-quarter of 13-year-olds lack adequate understanding of the content and procedures stressed in elementary school mathematics; nearly half of the 17-year-olds do not have the command of skills required to perform tasks with decimals, fractions and percents, or basic geometry and algebra.

The mathematics proficiency of 9- and 13-year-olds was higher in 1986 than in the first assessment in 1973. The performance of 9-year-olds remained quite stable across the 1970s.

-Forty-one percent of the 11th graders and 60 percent of the seventh graders in science classes report never being asked to write a science experiment independently.