The Surpreme Court, in an important expansion of last week's Bakke decision, yesterday rejected challenges to a major affirmative-action program adopted five years ago by the American Telephone & Telegraph Co. to end job discrimination.

The plan, negotiated under government pressure, was described at the time by a federal judge as the "largest and most impressive civil rights settlement" in U.S. history. It has led to the hiring, training and promotion of tens of thousands of women and blacks and other minority- group member within AT&T's 767,000-member work force.

In refusing to review a lower court's approval of the AT&T plan, the court rejected claims by three labor organizations that the plan violated valid collective-bargaining agreements covering seniority rights among workers and amounted to a kind of reverse discrimination.

The decision was one of four actions yesterday in which the court, winding up its current session, offered more clues to the implications of its Bakke decision upholding the principle of affirmative action while striking down rigid quotas.

Allan Bakke, who is white, contended that he was unconstitutionally denied admission to the University of California at Davis Medical School because of quotas reserving a specific number of places for blacks and other minority-group members. In a 5-to4 decision, the Supreme Court ordered the school to admit Bakke but also held as a general proposition that race can be a legitimate factor in screening applicants so long as it is not the sole factor.

This appeared to uphold most jobs as well as school affirmative-action programs, although other cases - particularly the AT&T case - were awaited for further clarification.

In the other three actions yesterday, the court:

Set aside a Los Angeles federal judge's ruling that Congress exceeded its authority in earmarking 10 percent of a $4 million public works law for businesses owned by minority-group members. The court instructed the judge to decide whether the case is now irrelevant because most of the money has been spent.

Ordered the 4th U.S. Circuit Court of Appeals to reexamine a ruling that struck down two University of North Carolina student government regulations assuring blacks and women a minimum level of representation in elective government posts. The court said the ruling should be reexamined in the light of the Bakke decision, implying that there may be grounds for approval of the regulations.

Agreed to decide whether an individual has the legal standing to sue for admission to a university on grounds of sex discrimination, which the 7th Circuit Court of Appeals said a private individual does not have the right to do.

The AT&T action was the most important, however, in fleshing out the Bakke ruling. It appeared to reinforce the government's powers, buttressed in the AT&T case by a judge-approved consent decree, to use goals, timetables and other prodding devices short of fixed quotas to end discriminatory practices in private industry.

There had been some doubt after a Supreme Court ruling 14 months ago in which the court held that seniority systems thay perpetuate the effects of past race or sex bias in hiring and promotion are not illegal unless the systems themselves were set up for discriminatory purposes.

Hence the AT&T case was awaited with considerable interest even after the court ruled in the Bakke college admissions last week. AT&T issued a statemen after the court's action saying it intends to continue its affirmative-action program even after the consent decree expires in January, although company officials said the provisions may be modified to conform with standards outlined in Bakke.

In the five years since the AT&T plan began, miniorities have nearly doubled their share of management jobs, from 4.6 percent to 8.7 percent, according to company statistics. Women have expanded their share of the top jobs from 22.5 percent to 27 percent.

In terms of overall employment at AT&T, minorities now account for 16.6 percent of all workers, up from 13.8 percent five years ago. IN the traditionally male crafts, women have moved from 2.8 percent to 8 percent, expanding by more than tenfold their share of outside-the-plant jobs like telephone lineman.

The consent decree, signed in January 1973 after months of negotiation between the company and various government agencies, required AT&T to establish goals and timetables for hiring, training and promoting women, blacks and other whom it had been accused of discriminating against in the past.

As of last year, the company reported yesterday, it was within 99.3 percent of its affirmative-action targets.

The agreement was not joined by unions representing telephone company workers, and the labor groups - the Communications Workers of America, the Telephone Coordinating Council of the International Brotherbood of Electrical Workers and the Alliance of Independent Telephone Unions - sued to block it.

Thet charged that the consent decree "constitutes an improper and illegal devise to remedy the effects of past discrimination."

Although the Supreme Court did not spell out its reasons for letting the plan stand, the 3rd U.S. Circuit Court of Appeals, in upholding the plan earlier this year, said:

"The use of employment goals and quotas admittedly involves tensions with . . . equal protection. But the remedy granted by the district court (in approving the plan) is permissible because it seems reasonably calculated to counteract the detrimental effects a particulary identifiable pattern of discrimination has had . . ."

In the Bakke decision, the Supreme Court indicated latitude will be allowed when remedies are aimed at specific acts of previous discrimination, which the government alleged had occured at AT&T before the consent decree.

Joseph Rauh, a Washington civil rights lawyer, said he interpreted the AT&T and the University of North Carolina actions as "rather favorable" to affirmative action and dismissed the order in the public works case as "just dd dismissed the order in the public works case as "just ducking."