The Los Angeles School Board voted today to dismantle a mandatory school busing desegregation program, a nationally unprecendented act expected to give a significant psychological boost to antibusing groups throughout the country.

By a 5-to-0 vote, the board told the school staff to be ready by April 20 to allow any child now being bused for racial balance to return to his or her neighborhood school.

Several teachers and parents, including some opponents of busing, said the early switch would bring chaos and asked the board to wait until the next school year, but board members said they thought parents ought to have a choice as soon as possible.

Some young, white supporters of the busing plan yelled "for shame, for shame!" from the audience as the board took the vote in a packed meeting hall. But the action reflected a popular political movement in the city and seemed likely to have influence even outside California.

"I think the time has come for a reevaluation of pinning all the hopes and dreams of the civil rights movement on busing," said school board president Roberta Weintraub in an interview. She said the board's success in overturning a two-year-old court ordered busing system might have the same nationwide impact as California's real estate tax reduction plan known as Proposition 13.

Some court-ordered desegregation schemes, such as in the southern California community of Inglewood, have been dismantled in the past, but only after enormous growths in minority populations have made them pointless. Attorneys for both sides said today this is the first case they know of in which busing for racial balance has been stopped while a substantial number of white pupils -- about 23 percent -- still remained in the schools.

Fred Okrand, an American Civil Liberties Union attorney who has represented parents and groups supporting the desegregation plan, said he would challenge the reversal in the courts. But most attorneys interviewed said they expect the U.S. Supreme Court to support the antibusing forces, at least in this case.

Despite the morale boost the Los Angeles case gives to antibusing forces outside California, it provides few if any helpful legal precedents. The program -- which buses 23,346 pupils throughout the crazy quilt school district, sometimes as far as 25 miles -- began because California's courts took an unusually broad view of what constitutes illegal school desegregation.

The courts held that segregation was illegal even if it was unintentional. A statewide antibusing measure, called Proposition 1, sought to bring California law into line with federal law, which generally permits court ordered busing only in cases where intentional segregation has been proven.

An appellate court ruled that Proposition 1 was constitutional and last week the state Supreme Court, dominated by judges usually in favor of desegregation, astounded all sides in the case by declining to review the appellate court's decision.

The school board's only black member, Rita Walters, said last week she was "appalled and horrified." She said the turnabout "built the walls of segregation ever higher and pulls us further apart." Today, Walters was in Washington on board business and did not vote.

The percentage of white pupils in the school district has been declining steadily, both before and after the busing plan began. Whites made up about 50 percent of students in 1970 when a Superior Court judge ruled the schools were segregated, and declined to 37 percent in 1976 and to 23 percent this year. Experts blame white flight from districts affected by the busing scheme for some of the decline, but say that declining birth rates and enormous increases in immigration of Hispanic families also helped bring the change.

Superior Court Judge Paul Egly, who has supervised the busing plan, has spoken publicly of its importance as a symbol of concern for the education of blacks and Hispanics in the city. Over the weekend, he denounced the reversal of the plan and today dropped out of any further consideration of the case.

From its beginnings in 1963, the effort to desegregate the Los Angeles city schools has followed a course as bizarre and unpredictable as the city itself. The crucial blow to the busing law was Proposition 1, which passed by a 2-to-1 statewide vote in 1979.But the scheme was severely handicapped from the start by having to serve a district that stretches in odd chunks and corridors more than 40 miles from the Pacific coast north to the Santa Susana mountains. The district encompasses the barrios of east Los Angeles and the spectacular seaside hills of the Pacific Palisades, the port neighborhoods of San Pedro and the tract homes of the San Fernando Valley.

After lengthy court battles, the school board proceeded in 1978 with a desegregation scheme for its half-million pupils. It relied on special "magnet" schools devoted to the arts or other special skills and on automatic transfers for any pupils in poor neighborhoods who wished to attend better equipped schools far from their homes -- all programs that are expected to continue.

But the plan also included a busing program affecting about 57,000 pupils -- 23,000 who rode buses, and 34,000 in the receiving schools. This last element galvanized the predominantly white northwest part of the city, particularly the San Fernando Valley, into a potent antibusing force.

One antibusing parent, Bobbi Fiedler, was first elected president of the school board and then elected as a Republican member of Congress. Elections soon gave the school board a strong antibusing majority.