Mayor Marion Barry received a detailed report yesterday on the city's handling of an 11-year-old Northwest Washington boy who was sexually assaulted in a D.C. Superior Court cell last year, but city officials said the report was confidential and refused to release it.

City attorneys "made it very clear that most of the report is covered by confidentiality statutes and could not be released," said Pauline Schneider, director of the mayor's office of intergovernmental relations.

Schneider said she does not know if or when the city will release details about the circumstances surrounding the arrest of the boy by police and his detention in the courthouse cell block.

"The only way I'm aware for the city to release it is if the boy's family went to court to lift the confidentiality and the court issued an order," Schneider said.

The boy, who had no previous criminal record and is now 12, was attacked May 24 by 14-year-old and 17-year-old cellmates who forced him to commit sodomy. He was later found to have contracted syphilis.

Police officials said that in most cases children that age with no previous trouble with the law would not be taken to court.

On Tuesday, Barry said he was "awfully concerned" about the incident and that he wanted "to find out what happened, why it happened and who made what decisions."

Barry later met with top officials of the police department and the corporation counsel's office. The corporation counsel's office had filed a petition charging the boy with assaulting a playmate with a baseball bat after it learned that the 11-year-old had been attacked in the cell.

Schneider said the mayor received reports from police and city prosecutors detailing the incident. "He the mayor made it very clear that he would like to have something that could be released."

The boy's family has told court officials and officials at Children's Hospital, where the youth continues to receive psychiatric treatment, that they would like to have the confidentiality provision lifted.

Asked to explain why the city insists on keeping the details confidential, Schneider said, "If you read the statute, it covers government officials. It doesn't cover families."

Meanwhile, the chief of juvenile justice programs in the Reagan administration came under sharp attack yesterday by House Democrats who said the D.C. case shows why such federal programs should not be halted.

"That this happened here in the seat of government, what type of signal does that send to the other states?" asked Rep. Dale E. Kildee (D-Mich.), chairman of the human resources subcommittee of the House Education and Labor Committee.

Treatment of jailed youths in the District "should be an example of how we protect the dignity of every individual," Kildee said. "That youth's dignity was assaulted."

Alfred S. Regnery, chief of the Office of Juvenile Justice in the Justice Department, defended an administration decision not to seek further funding for federal juvenile justice programs. The programs are designed, in part, to encourage states to find alternatives to jailing truants, runaways and youths accused of minor crimes.