Educators here in the nation's second-largest public school system are learning a hard lesson of their own these days: Ending social promotion is much easier said than done.

Faced with the prospect of flunking several hundred thousand children, and suffering from dire shortages of classroom space and qualified teachers, the Los Angeles school system has just decided for the second time in recent months to limit its plan to keep students with failing marks in the same grade until they pass.

Now, under guidelines that the school system's leaders revamped this week, the looming crackdown against social promotion will only affect students who are failing English, not other core subjects such as math. And even that pool will be limited. Late last year, the school system decided to impose tougher standards for passing to the next grade only on second-graders and eighth-graders.

"We're facing a huge logistical problem," Shel Erlich, a spokesman for the Los Angeles Unified School District, said. "If we hold so many people back, where are we going to put them all? We don't have a solution yet, so we're not going to do everything we would want to do all at once."

The predicament unfolding in Los Angeles over ending social promotion is apparent, to different extremes, in schools across the country. Raising academic standards, and retaining students who do not meet them, is the budding reform of the moment in public schools, but the movement is creating enormous political uproar and seismic changes of habit in classrooms--especially in large urban school districts. In the past few years, two dozen major cities have ended social promotions and a third of the states--from California to Virginia--have moved away from the practice.

In cities such as Chicago and the District of Columbia, both of which have long had deeply troubled public school systems, it has forced thousands more failing students into summer school for the first time.

In Los Angeles, which recently ousted its schools superintendent, the drive to end social promotion is also colliding with two other powerful forces: Soaring enrollment and another urgent campaign to reduce class sizes. California is ordering all of its schools to end social promotion in the next two years at the same time it is mandating smaller classes. And in many places the reality of trying to accomplish both goals is getting controversial and complex.

"No one is retreating from the notion of ending social promotion, but everyone is struggling with how to implement it," said Michael Casserly, the executive director of the Council of Great City Schools, an advocacy group for large urban public school systems nationwide. "It is raising a profound set of questions and choices. And people are realizing that if it's done wrong or rushed, it's a formula for social chaos."

School officials here recently estimated that if they took a swift and hard line on ending social promotion, as many as half of the city's 700,000 students could be forced to repeat a grade. That projected rate is the highest in the nation, educators say. One of many reasons for it is that more than 50 percent of public school students in this multicultural metropolis are not yet fluent in English, which makes meeting the system's academic standards difficult for many of them.

Crowding is another crisis. In the past five years, enrollment in the public schools of Los Angeles has increased by 75,000 students, and no end to the boom is in sight. Some educators here say the public schools simply do not have the room or the manpower to keep failing students in the same grade. The teachers union in Los Angeles, which has 40,000 members, is also pressuring the district to give school faculty more training to deal with the sensitive issue of which students should pass or fail.

Together, those pressures have prompted the acting schools superintendent, Ramon C. Cortines, to revise the earlier and far more ambitious plan Los Angeles has to end social promotion. The goal, his aides say, is to try to extend the tougher academic requirements gradually to other grades and other subjects.

"For now, we have to be realistic," said Stephanie Brady, an adviser to Cortines. "We have to tell our schools that reading is what we have to focus on, because that's the one skill that affects everything."