In the Wilmington, Del., metropolitan area yesterday, 34,000 students - more than half the public school on the first day of a new city-suburb desegregation plan.

In Los Angeles, the nation's second-largest school system, up to 60,000 students will be bused today under a plan approved by a state court.

In Seattle, once teachers end a strike now five days old, about 12,500 students are expected to ride buses under a desegregation plan conceived by the local school board.

As these and a handful of other examples attest, school desegregation remains - however faintly - a live issue in America.

Particularly in the northern cities which now have the largest concentrations of blacks, courts are still churning out occasional desegregation decrees.

But they are coming slowly, and the South now has far and away the least segregated public schools in the country.

Only 17 percent of black public school students in the Southeast, for example, attend schools that the federal government regards as segregated.

By comparison, 39 percent of black public school students in the Northeast attend segregated institutions, and 47 percent of all black public schools students in the northcentral region attend schools deemed segregated by the government.

A segregated school is one in which the racial ratio differs from that of the school district by more than 20 percent, the government says. And according to latest available figures (1976) offered by the Department of Health, Education and Welfare's Office for Civil Rights (OCR), most such schools are now in the North.

"The level of segregation is considerably higher in the Northeast and the Northcentral regions than it is elsewhere," according to a current OCR "limited analysis" of the subject. "But even in the region with the lowest average level, the Southeast, the level of segregation is noticeable," the report said.

Experts give several reasons - all familiar - for this continuing racial isolation in the schools of the North.

One is essentially logistical. Systems like New York or Los Angeles are vast; tens of thousands of children would have to be bused to desegregated them, and there are not always enough whites left to go around.

Another is legal. The Supreme Court has resisted ordering desegregation of northern school districts except where racial separation can be shown to be the result of official intent, which is frequently difficult to prove. For similar reasons, it has been reluctant to order city-suburb using as a way of dealing with white flight from central cities. The Wilmington plan is an exception to this general reluctance.

A third factor is loss of will among northern liberals who supported desegregation in the South but flinched as it came to their own school districts, and among civil rights groups that have turned their attention to other issues.

That is why desegregation experts such as Gary Orfield at the University of Illinois say prospects for effectively desegregating schools in the North's large urban areas are "grim as hell."

Chicago is an example, said Orfield, who has done an exhaustive study of the issue in his book, "Must We Bus?" In that Midwest city, segregation in the school system approaches "racial apartheid," Orfield says.

He said he doesn't expect a substantial reduction in Chicago school segregation this year, even though the city has started a "voluntary transfer" program, involving about 1,319 students.

Instead, Orfield predicted that "if things continue the way they are," Chicago's public school population will be less than 15 percent white by 1988, eliminating any chances of significant school desegregation. Whites now account for about 23 percent of Chicago's public school students.

"For much of northern urban society in this country, it's going to be the same thing as it was in the Old South, and that's going to poison the whole system," said Orfield.

Blacks constitute 6.77 million (15.5 percent) of the total public school enrollment. Of their number, about 30 percent attend segregated schools, mostly in the Northeast, Northcentral region and West.

Hispanics account for 2.81 million (6.42 percent) of the nation's public school students. Of their number, 24 percent attend segregated schools, mostly in the Northeast, Southeast and West.

Asian Americans account for 1.22 percent and American Indians make up less than 1 percent of public school students in the country. Some 3 percent of Asian Americans go to segregated schools, mostly in the West; 11 percent of American Indians attend segregated schools, mostly in the Northcentral, Southeast and West.

Central city schools enroll about 51 percent of the nation's black and Hispanic students, a fact which plays a large role in "white flight" and continued resistance to desegregation programs, according to desegregation experts.

"Many whites can't conceive of anything that is all or mostly black as being good," Orfield said. So he said, it is not surprising to find strong resistance to massive desegregation plans in places like Columbus, Ohio, which was scheduled to begin busing 37,000 public school students this fall. Opponents of the $6.3 million Columbus plan went before Supreme Court Justice William Rehnquist last month and won an "emergency appeal" blocking the plan's implementation.

In Dallas, school officials have been fighting desegregation orders since 1955, while the white population of the city's school system has fallen to 35 percent. The system now has 140,000 students and is fighting its seventh order to end or justify the existence of what the 5th U.S. Circuit Court of Appeals calls its "one-race" schools.

Still, according to Norman Chachkin of the Washington-based Lawyers' Committee for Civil Rights, desegregation is enjoying more wins than losses.

In Boston, scene of much busing-related violence in 1974, school opened this year without incidence, he noted. Last month's decision by U.S. District Court Judge W. Arthur Garrity to free South Boston High School - the symbol of Boston's desegregation resistance - from federal receivership is "cause for hope," he said.

In the Wilmington area, where 11 separate school districts were merged as part of the desegregation plan, officials were jubliant about the "uneventful" opening of schools yesterday.

"I'm delighted at what's going on," school board member Earl J. Reed told Associated Press. "Everything is going smoothly."

Chachkin said he expects a similar reaction in Los Angeles today, although police have made preparations for possible trouble.

"There is a scenario to these things," he said. First the crisis; then there's acceptance."