LOS ANGELES, OCT. 13 -- Responding to a flood of immigrant children, the Los Angeles city school board has voted to put all its schools on a year-round schedule -- effectively doubling the number of year-round classes in the country and signaling a growing education crisis in much of the urban West.

The 4-to-3 vote Monday night, after hours of impassioned debate and years of political foot-dragging, would make Los Angeles the largest year-round school district in the country when the plan takes effect in July 1989. Supporters say the district would become a giant laboratory to test growing interest in the year-round system as a way to relieve crowding in order to facilitate learning.

Parents and students in the predominantly Anglo neighborhoods of Los Angeles' San Fernando Valley remained unenthusiastic today. The valley is one of the few places in the city without crowded schools. Many parents there have threatened to put their children into private schools if the new system cuts into plans for summer camp or long summer trips.

Although the exact year-round schedule has not been determined, school officials here have suggested that each student would take a one-month vacation -- in August, December and April -- at the end of three three-month semesters. The three-month summer vacation originally designed to allow youngsters to work in the fields would be abolished, although a school spokesman said a compromise summer vacation longer than one month was still possible.

"It's not fair," said Herlette Braxton, 16, a junior at James Monroe High School in the valley community of Sepulveda. "You might want a a job in the summer, but who's going to hire you for four weeks? It's stupid."

"You put schools year-round on a single track and you won't have to worry about overcrowding," said Roberta Weintraub, a school board member representing part of the San Fernando Valley. "There's going to be a massive pullout starting today."

Critics were particularly enraged to discover that district executives were pushing the plan despite a sharp drop in the growth rate of the student population this fall. Only 2,000 of an expected 12,000 new pupils showed up in September, but school officials wrote this off as a temporary phenomenon. They said they thought the new federal immigration law had made undocumented aliens afraid to register their children.

Officials said they expect total enrollment, now about 600,000, to reach more than 700,000 in 1996. Without a year-round system, the district would have to build nine elementary schools, one junior high and one high school every year to keep pace, the officials said.

"People are gradually beginning to realize that it does not make sense to keep kids away from formal education for three months every year," said Charles Ballinger, executive secretary of the San Diego-based National Association for Year-Round Education. Although most of the approximately 350,000 children now in year-round classes were put there because of inadequate classroom space, some less-crowded districts are also using the system because it helps students retain what they have learned without a long summer break, he said.

Ballinger said the year-round concept gained brief popularity in the suburban boom of the late 1960s and early 1970s, when communities such as Dale City in Prince William County, Va., could not build schools fast enough. Year-round classes then declined as school populations dropped -- ending in Prince William in 1981 -- but the immigrant surge and a shortage of school funds in the West has reversed the trend.

Parental and student pressure until now had limited the year-round system to those parts of Los Angeles most affected by the flood of immigrants from Latin America and Asia. About 150,000 Los Angeles children now attend year-round classes, in a confusing array of systems that the new plan is designed to consolidate.

Some school board members who voted for the year-round plan said it was unfair and unwieldy to continue to impose it selectively. "Until now, in my opinion, this district has been practicing a policy of containment," said Jackie Goldberg, whose Hollywood-Wilshire district already has many year-round schools. She said some schools have been kept "isolated by geography {and} class" and some parents have "the mistaken impression that overcrowding is not a problem because it is not on my block."

Reacting to suggestions that Valley voters might try to take their schools out of the city system, board president Rita Walters said "we cannot operate a district based on threats."

School officials and school board supporters said the district-wide year-round system would reduce inconvenience for parents with children in different schools and force summer camps, recreational programs and child-care agencies to match their schedules to the new plan.

In overcrowded schools, students are expected to be put on a "multi-track" system. Students will be divided into four groups, called "tracks," and attend school on staggered schedules. At any one time, the students in at least one of four tracks would be on vacation, reducing the demand on classroom space by one-fourth.

In less crowded schools, students would use a "single-track" system, in which all would take shorter vacations at the same time, three times a year. Officials said it was important that all schools adopt the same general vacation plan so that transfers would be possible and siblings at different schools could keep the same schedule.