Half of the special education students who attend private schools at the District's expense have failed to prove city residency and have only a few weeks before the school system stops paying their tuition bills, which amount to $35.6 million a year, officials said yesterday.

The warning to special education families by D.C. School Superintendent Arlene Ackerman's administration is part of a broad effort by school and city officials to ensure that children being educated with District money actually live in the city, as required by D.C. law.

As elementary as that task might seem, this is the first year in recent memory that officials have tried to get tough about residency with special education students, and only the second year that they have done so with the rest of the student body. Complicating matters, many students in the District's fledgling charter schools--which are publicly funded but operate outside Ackerman's control--have failed to prove residency.

In the past, records were in such chaos that officials did not even know how many students they had, much less where they lived. And school officials never bothered to monitor the students who had been placed in private special education schools by courts and administrative hearing officers after the school system failed to provide vital services.

Ackerman, who has been superintendent for 1 1/2 years, has vowed to clean up the records and require residency documentation. She also has begun reforms in special education, including monitoring the progress of private school students.

At a news conference yesterday, Assistant Superintendent Ralph Neal said that more than 69,650 students in neighborhood schools have verified their residency, while 1,068 still must do so. Of the more than 70,000 students in neighborhood schools, about 7,600 are special education students. Virtually all have proved residency, special education director Anne Gay said.

But, officials said, of the 238 D.C. special education students attending private residential schools across the country, 178 still must prove D.C. residency. Of 1,795 students in private day schools in this region, 890 must do so.

The average cost of a private special education day school is $30,000 a year, and the average residential program costs $50,000.

Some special education activists said they doubted the school system had done all that it could to reach out to parents, and did not believe many of these students were non-D.C. residents.

"I think they are probably making much out of nothing," said lawyer Margy Kohn. "Many special education parents don't realize that they have the same obligation to verify that others have," she said, in part because their children are placed in private schools by judges and hearing officers who ask residency questions during the proceedings.

"I strongly suspect they will find very, very few nonresidents being educated in private schools because of the amount of scrutiny given to those placements," said lawyer Beth Goodman.

Neal said the system has written and phoned parents repeatedly since last summer. He sent a final letter Nov. 5 to parents and guardians of students in residential special education schools, with a verification deadline of Nov. 30. A letter was sent Monday to private day school special education families with a Dec. 15 deadline.

After those deadlines, "we will stop making those tuititon payments," Deputy Superintendent Elois Brooks said yesterday.

Brooks said that she did not know how many students live outside the city but that families had had many opportunities to prove residency.

"We have to make sure we are providing services to District of Columbia students," Brooks said. "We have done everything humanly possible to try to get these families to prove" residency.

CAPTION: Superintendent Arlene Ackerman is trying to clean up school records.