The D.C. school system has begun preparing to instruct the youngest and most helpless victims of the city's drug crisis, who now are arriving in classrooms and whose numbers are expected to rise dramatically over the next few years.

School officials for the first time are using classes of elementary school students as settings to learn the problems of children born to drug-addicted mothers, believed to be one of the city's fastest growing populations.

Six elementary school classrooms, which the school system is not naming to protect the privacy of families participating, became sites this month where social workers, psychologists and speech pathologists are observing and aiding students who were exposed to drugs in their mothers' wombs.

The effort will continue until the school year ends in June; officials then intend to use their research to develop a citywide strategy for helping students overcome the learning problems that prenatal exposure to drugs such as crack cocaine apparently create.

Without early and extensive intervention, school officials said, the chances are small that these students will succeed academically.

A primary goal of the program is to find out more precisely what obstacles these students will face in classrooms and give teachers some clues on how to help them, said Maurice Sykes, the school official heading the project.

At the same time, according to Sykes, the researchers hope to prevent the mislabeling of students who are slow learners but are not the children of drug-addicted mothers.

School leaders say they do not know exactly how many students suffer from such exposure, but the figure is widely estimated to be in the hundreds -- and bound to get worse.

D.C. General Hospital estimates that one of every five pregnant mothers it treats is addicted to drugs.

"This problem is going to be immense," said School Superintendent Andrew E. Jenkins. "It's going to put a very heavy burden on us."

School systems in many of the country's largest cities are expressing similar fears as they greet the first generation of children who were born amid the crack epidemic of the past five years.

The ailments these children bring to classrooms are not yet clear, but there is strong evidence that many of them have speech defects, retarded motor skills and poor attention spans.

In the District, educators and medical officials began work on the new project earlier this year. Four of the six classrooms involved in the effort are being used to observe the behavior of students who the District is certain were born to drug-addicted mothers.

In the other two classrooms, a team of medical and social work professionals is intervening in the lives of the students and their families.

The students are not being taught differently than other D.C. elementary school students, but a social worker and psychologist are working with their teachers to improve academic performance.

"We are really at the beginning of this kind of work," said Johanna Ferman, executive director of the D.C. Institute of Mental Health, who helped draw up the school system's plan. "But this is an important first step."

School officials said they found the first evidence of large numbers of students with prenatal exposure to drugs in pre-kindergarten and kindergarten classes last year.

That coincides with the arrival of crack on District streets about five years ago, and with the surge in cocaine and PCP use.

In 1985, D.C. General Hospital officials concluded that about 6 percent of the mothers who gave birth there were addicted to drugs. By 1988, the figure had leaped to 20 percent.

Greater Southeast Community Hospital estimates that nearly 40 percent of the mothers who give birth there are addicted to drugs.

"The larger numbers are coming," said school board member Karen Shook (At-large), who heads the school board committee supervising the project. "We have to be prepared."