Attorney General Griffin B. Bell said today that he accepts a controversial stand limiting school busing, which was taken by the Justice Department in the Ford and Nixon administrations, because the Supreme Court has made it the law of the land.

Civil rights advocates are expected to dispute his assertion on the gournd that the court has yet to agree or disagree with the department position that the scope of remedies for segregated schooling should be no greater than the proved wrongs.

Bell, questioned on "Face the Nation" (CBS, WTOP) and in a later press conference here at the midyear meeting of the American Bar Association, also said that he hopes to make public shortly a 148-page report on a department assessment of the FBI's investigation of the assassination of Martin Luther King Jr. in 1968.

Bell said that the report could lead some persons to the conclusion that there is no evidence of a conspiracy to kill King, while leading others - because of unanswered questions - to wonder if there was a conspiracy.

"I want everybody to be able to read the report and make up their own minds on it," he said.

United Press International reported on Feb. 2 that the report concluded that James Earl Ray was the lone assassin.

Sources told a Washington Post reporter the same day that the report also concluded that the FBI, which conducted a lengthy undercover campaign ot discredit King, had made an adequate professional investigation despite shortcomings in its performance.

Questioned about the status of the law on school busing, Bell told reporters that the Supreme Court has interpreted the law in accordance witht he views of the two previous administrations and that he intends to leave the situation as is.

He said he based his appraisal on a Supreme Court action in December concerning an order to the public schools of Austin, Tex., to undertake system-wide busing involving Mexican-Americans as well as blacks.

Without issuing an opinion, the high court sent the case back to a lower court with instructions to review it in light of an earlier decision that racial discrimination must be shown to have been intentional if it is to be held unconstitutional.

In doing so, the high court did not pull back from a decision in an earlier case from Denver that federal judges may presume that a segregative intent proved in parts of a school system pervades the entire system.

Justice Lewis F. Powell Jr., in a statement joined by Justice William H. Rehnquist, objected to the Austin busing order on the ground that it fashioned a remedy that exceeded the problem.

Bell said he believes that the objection by the two justices was implicitly the view of a majority of the nine-justice court, although he acknowledged he might be wrong because his contention was based on his perception of practical reality rather than on hard evidence.

He also said that by its action in the Austin case the high court had "put a gloss" on the Denver ruling that trims the discretion afforded judges in fashioning desegregation remedies.

Whether judges' discretion will be cut back may be known later this year when the court acts in a possibly crucial busing case from Dayton. In that case, a federal judge has required enrollment of black pupils in each public school in a proportion diverging by a no more than 15 percentage points from the proportion of blacks in the city population.

Bell conceded that he did not know that the court had agreed in January to review the Dayton case. He noted that he designated solicitor general, Wade H. McCree Jr., will have to be disqualified that case because he is a judge on the appeals court that has had jurisdiction over it.

On another subject, Bell said he had rejected a suggestion by some aides that he falsely promise immunity from federal prosecution to Anthony G. Kiritsis, who last week threatened to kill an Indianapolis mortgage banker held hostage with a shotgun attached to his neck.

"I said I don't think the government ought to lie," Bell recalled. He said his main fear was that plane hijackings might occur in which large numbers of persons would be imperiled because hijackers wouldn't believe that prosecutors' promises of immunity would be honored.

At the same time, Bell said, Indianapolis authorities had been "absolutely right," legally and morally, in securing the banker's release by falsely promising immunity from state prosecution. He added that it is a sound principle that a promise under duress is not binding.