While public school enrollment has declined nationwide during the past three years, enrollment in private schools has shown modest increases for the first time in several decades, according to a study to be made public today by the Department of Education. The increases reflect the dramatic growth of fundamentalist Christian schools, Education Department officials said.

The study by the department's National Center of Education Statistics indicates that in 1983, 12 percent of all students in grades one through 12 were enrolled in private schools, compared with 10.5 percent three years earlier. The increased share of students in private schools indicates a growth from 4.6 million to 5 million students.

During the same time period, the total enrollment of students in the first through 12th grades dropped from 44 million to 41.6 million.

While officials were guarded in interpreting the report, the numbers were seen by some representatives of private schools as an indication that private education had become more popular.

"Maybe more people have discovered the non-public sector," said Anne Rosenfeld, a spokesman for the National Association of Independent Schools. "The morale has not been good in public education recently. I'm sure that doesn't help their enrollment picture. A lot of people are applying a consumer attitude, a shop-around attitude to schools."

The National Education Association, however, which represents public school teachers, said the figures should not be interpreted as a reversal in sentiment about public schools.

"It's certainly not a significant shift . . . . It's but a blip, that we would have to wait and see," said NEA spokesman Howard Carroll.

Charles J. O'Malley, executive assistant for private education at the Department of Education, also said the figures were less a rejection of public education than an indication of the search for religious education. "People are concerned with getting moral education they can't receive in the public schools," he said. "With the economy leveling off, more people can afford church-related schools."

During the same time period, the report showed, there was a 19 percent increase in enrollment in private preschools and kindergartens. But that growth probably reflects changing demographics and the trend of enrolling young children in preschools, few of which are public, according to Thomas Snyder, a statistician for the National Center of Education Statistics.

Snyder cautioned against reading too much into the study because of a change in methodology that makes it difficult to compare numbers before 1980. The study, he said, is more accurate than previous research on the number of schools, and it "does reflect some growth, but you've got to be careful about drawing any grand extrapolations from it."

Although the study did not break down non-Catholic schools according to religious affiliation, officials said much of the growth should be attributed to fundamentalist Christian schools. The report gives some validity to claims by Christian school associations that three new fundamentalist schools are opening daily, said Norman D. Beller, an official at the statistics center.

At the same time, the report stated, Catholic school enrollment has continued to decline, from 3.4 million to 3.2 million, and the number of private schools has grown from 24,500 to 27,700 since 1980.