While American elementary school students now perform better on most tests, the upswing began with children born in 1962 -- long before the recent spate of education reforms brought tougher standards into the classroom -- according to a comprehensive government study.

The report, "Trends in Educational Achievement" written by the Congressional Budget Office, suggests that the current increase in student test scores -- coming after a sharp dip in the 1960s and early 1970s -- may be due primarily to a new generation of children coming of age who were not subjected to the same societal pressures as the older children.

Many school administrators and Reagan administration officials have hailed the turnaround in student reading and mathematical skills as proof-positive that the reform movement had already taken root, barely three years after the widely publicized report "A Nation at Risk" warned of a "rising tide of mediocrity" in U.S. education and prescribed a back-to-basics remedy.

Since the publication of that presidential commission's report in 1983, virtually every state has taken up the call to reform its schools, with most states raising teacher pay, initiating tougher high school graduation requirements and instituting stricter screening for new teachers.

Last September, when the average scores on the Scholastic Aptitude Test (SAT) posted the largest single-year gain in 22 years, Education Secretary William J. Bennett said, "Bravo! We begin to see here the impact of the reform movement of the past several years."

But the new CBO report says that the turnaround was under way well before the commission report was published and before the reform movement was launched.

"Whether or not these reforms have contributed is unknown," said Daniel Koretz, CBO analyst who prepared the new study. "What is clear is you can't claim that these reforms initiated the upturn."

The CBO report, by showing an educational upswing beginning with children born in 1962, also may provide ammunition for supporters of federally funded programs for the disadvantaged that have been targeted for cutbacks. Those children responsible for the improved performance would have entered school just as many of those federal programs were getting under way.

"It's a real plaudit for Chapter I and Title I," said Scott Widmeyer, spokesman for the American Federation of Teachers. "It shows that those special programs are useful."

The CBO found that "the overall decline in test scores generally ended with . . . children born around 1962 and 1963 -- that is, with the [children] that entered school in the late 1960s." Thus, the report says, "the decline's end first appeared in tests administered in the upper elementary grades in the mid-1970s.

"This pattern, however, has gained relatively little attention," the report says. "Perhaps because of the greater notice accorded to tests at the senior high school level, there has been a widespread misconception that the decline only ended within the past few years."

The CBO report is expected to add a controversial new element to the reform debate as it enters a more difficult phase. Until now, reform has been aimed at high schools, centering on the decline in basic skills and looking to what could simplistically be called "quick-fix" solutions, such as increasing the number of English and math courses required to graduate.

In this second phase, the reform movement is shifting to less-tangible and potentially more costly areas, such as improving the climate for teaching and learning, education experts said.

Koretz said the new study challenges the conventional wisdom that schools themselves "did something" -- something bad to bring about the earlier decline, and now something good to initiate the upswing.

If future indicators show the decline in test scores was limited to approximately one generation -- the children in school from the mid-1960s through the early 1970s -- then the cause may have been "broad societal changes" going on outside the classroom, he said.

The 165-page report was prepared at the request of the Senate subcommittee on education, arts and the humanities. Koretz said he used about 15 separate tests as indicators, including the SAT, the American College Testing Program (ACT), the National Assessment of Educational Progress and several state tests.