In the past few years, reformers have embraced a disarmingly simple idea for fixing schools: Why not actually flunk those students who don't earn passing grades? Both Democrats and Republicans have begun attacking the practice of "social promotion"—shuttling bad students to the next grade, advancing them with peers even if they are failing. Make F truly mean failure, the movement says.

Last week in Los Angeles, the reformers learned just how ornery the current system can be. According to a plan released Tuesday by the L.A. school district, ending social promotion there will take at least four years, could cost hundreds of millions of dollars—and probably would require flunking about half the district's students. That's a pessimistic assessment, but it's not just bureaucrats' caterwauling. Rather, L.A. school superintendent Ruben Zacarias was an eager convert to the crusade against social promotion. In February he unveiled an ambitious plan to end unwarranted promotions in five grades during the 1999-2000 school year—a full year ahead of the timetable set by a state law.

At the time, Zacarias acknowledged that his goal would be hard to meet. He estimated that as many as 6 of every 10 students would flunk if they had to advance on merit. Zacarias wanted to spend $140 million in the first year alone to help these kids. Why so much? Because a mountain of research shows that ending social promotion doesn't work if it just means more Fs. Kids who are simply forced to repeat grades over and over usually don't improve academically and often drop out. Zacarias wanted more tutoring, summer school and intensive-learning classes. Unqualified students wouldn't rise to the next grade; nor would they be doomed to redo work they already failed. It was a forward-looking plan that Zacarias, 70, didn't have the clout to enact. He wasn't popular enough—the school board recently bought out his contract after a bitter power struggle—but even fellow reformers think his plan was too much, too soon. Says board member David Tokofsky: "You've got the unions who want their say. And, of course, there's the facilities issue: Where do you send all these eighth-graders if you can't send them to high school?" The district now says it will stop advancing low-achieving students only in two grades (second and eighth), and it will begin next year.

Los Angeles isn't the only place that has run into roadblocks while trying to end social promotion. In New York City, some advocates have said in lawsuits that parents weren't notified early enough that their kids were flunking. And in Chicago, which led the nation on the issue, a parents' group has filed civil rights complaints alleging that the promotion crackdown holds back a disproportionate number of black and Latino kids.

Still, the war on social promotion could have one salutary consequence: if every school district takes L.A.'s approach, struggling students will get a lot more teaching help, not just a kick in the rear as they finish another unproductive school year.