Today's American teen-ager - reputed to be the best-taught and best-informed in our politically endangered species list.

Most teen-agers still can name their president, but after that their knowledge and understanding of government and politics falls off dramatically.

Fewer than half can name one of their senators or their representatives in the House.About a third don't know that a senator is elected. Fewer than half know that the Senate must confirm a nomination to the Supreme Court.

More than a third don't believe a newspaper should be allowed to publish criticism of elected officials. A fourth don't know the Senate is part of Congress. A third don't know the Constitution prescribes their civil rights.

Those and other findings were reported yesterday by the federal government's National Assessment of Education Progress, a nationwide testing project.

More startling, NAEP officials said, is that the latest round of testing of 145,000 youngsters shows a decline from six years ago - even in an era of seemingly heightened American political perceptions.

Marie D. Eldridge, administrator of the Department of Health, Education and Welfare's National Center for Education Statistics, the overseer of NAEP, called the new report "an extremely important document."

She and other federal officials said at a press conference that the decline in test results cannot be blamed solely on the schools, although enrollment in social studies in public schools is falling steadily.

"Political knowledge and attitudes are not acquires solely through the academic process," Eldridge said. "The media, the community and the family can certainly be credited with substantial influence."

NAEP, which is run by the Education Commission of the States in Denver, is an attempt to depart from traditional testing methods to give a clearer notion of how well and how much students learn.

The organization conducts annual surveys nationwide to test student performance in the basic academic areas and to measure change over time.

The results announced yesterday were the product of tests carried out in 1975 and 1976 among selected 17-and 13-year-old students, and, in some areas, 9-year-old pupils.

Questions asked in the original survey of political knowledge and attitudes in 1969-70 and 1971-72 were repeated.

Roy H. Forbes, NAEP director, said the students' test scores declined in each of the five testing categories - structure and function of government, political process, constitutional rights, respect for others and international affairs.

Though NAEP makes no claim that its findings are the last word, it said other generalizations could be made from the results:

Boys and girls did about equally well in responding. Suburban youngsters performed better than inner-city ones. Those from high-income areas performed above the national level. Pupils from the Northeast and the central part of the country scored participation in the political process.

[TEXT OMITTED FROM SOURCE] higher. Whites did better than blacks. Children of more-educated parents scored higher than any others.

Despite the overall decline of scores in most subject areas, there were what Forbes called "a few bright spots" and what Eldridge described as "some reasons for encouragement."

For example, student support for constitutional rights stayed at about the same level. Improved scores turned up on questions reflecting concern for others' rights - especially minorities, understanding the problems of the poor and an awareness of the need for world peace.

On one item, dealing with the legal rights of the accused, the 13-year-old group improved its score by 20 percentage points. NAEP theorized that television shows on police and lawyers may have been an influence.

Three-fourths of the teen-agers, about the same as before, expressed a willingness to live near, worship with and have social ties with persons of another race. The number of 13-year-olds with positive racial attitudes was about the same, but the percentage of 17-year-old increased markedly.

As part of yesterday's press conference, the National Endowment for the Humanities announced a $100,000 grant to the Alliance for Citizen Education - which includes the AFL-CIO, the National Council of Churches, the Robert F. Kennedy Memorial Foundation and the National Association of Neighborhoods - for a series of regional conferences to promote citizen [TEXT OMITTED FROM SOURCE]