Carolina Acevedo stood in a swirl of students in the cafeteria of the District's Roosevelt High School and, clutching a handful of stickers each marked with a month, collared her classmates one by one.

Switching from Spanish to English and back again, she asked each one when they were born. Then she cajoled, wheedled and, occasionally, just pushed students in the direction of tables marked by month. She was trying to break up their normal seating patterns, hoping that Julys would bond with Julys, Decembers with Decembers.

Across the country this week, schools tried to shake up the invisible boundaries -- race, class, clothes, sports -- that come into stark relief in the shifting social structure of the school cafeteria. Mix It Up was the first nationwide activity sponsored by Teaching Tolerance, a program of the Alabama-based Southern Poverty Law Center.

Prince William County's New Dominion School, an alternative school for sixth-, seventh- and eighth-graders, conducted its experiment yesterday and had such success that it will be repeated once a month.

At Roosevelt on Wednesday, trying to budge teenagers from their self-assigned tables was a little like trying to pull up trees.

"Why you gotta segregate us?" one teenager demanded. "I got my own little corner, man." Others came back for different stickers until they were covered with five or six, a passport that allowed them to float through the cafeteria until they were sitting with their friends again.

Others ignored the whole thing as they dug into a lunch of turkey and stuffing.

Acevedo, a 17-year-old senior, surveyed the scene a half-hour into the lunch period. Latino students were starting to drift to one side of the cafeteria, black students to the other -- as usual.

"There's no major beef between the racial groups. But there's no mingling either," Acevedo said. "I think it's because it's lunch, and because it's a time for them to be with their friends."

But at some tables, it worked.

"I actually liked it," said junior Latoyya Valteau, 16. "These were some people from Guatemala -- I can't tell you their name because a sister can't pronounce, okay? But they started to open up, once you told them you were nice and not up in their face."

Targeting the lunchroom seemed an obvious choice to the law center, which is best known for tracking hate groups and taking legal action against white supremacists. Everyone knows what it's like when the jocks sit with the jocks, the cheerleaders with the cheerleaders, said Jennifer Smith-Holladay, director of Tolerance.org, another law center program.

"The social boundaries that exist in our schools are in many ways just as insurmountable as Jim Crow legal boundaries," she said.

Cliques aren't always bad, educators say. For many kids, grouping themselves by cliques is a natural way to navigate the enormous world of high school.

"For the majority of kids, it's the way they get along," said James Youniss, a Catholic University professor who has studied youth and child development issues for 40 years. "The popular kids, they're always aloof. But there's lots of other groups that are not designed for exclusion. It is usually fairly benign and actually quite functional."

But exclusion can still sting. And "when they feel they need to look down, rather than look across," that's when behavior can be harmful, said Manny Bartolotta, counselor for the West Potomac Academy, a career-education program based at West Potomac High School in Fairfax County.

Mix It Up started with a modest goal of enlisting 1,900 schools nationwide, but early returns indicate that as many as 3,000 schools will participate.

A program such as Mix It Up can show real changes if it's not limited to just lunchtime, Bartolotta said. "To the extent that the entire staff makes a big deal about it, it can work," he said. "You can point out how limiting it is to limit your social circle."

At New Dominion, where 60 sixth-, seventh- and eighth-graders are enrolled because of behavioral problems at their base schools, pupils usually spend the entire day, including lunch period, in their own classroom. Yesterday, teacher Lynn Sillitoe made a random lunchtime seating chart in a common area and assigned one student at each table to spark conversation if it started lagging.

"Does everybody know each other at the table?" asked Desiree Fowler, 14, an eighth-grader who took the job seriously. "What's your favorite class here?"

New Dominion Principal Ed Doyle said he had a few misgivings about the experiment. "Even adults don't like to sit with people they don't know," he said. But it was such a success that the staff decided to do it once a month.

"I had a student at my table," Desiree said, "he thinks he's big, he doesn't like to talk. But you found out that he likes science, he likes to experiment on things. . . . He said New Dominion changed him. He's not the person I thought he was."

James Jerman, 11, talks to Lenyawnia Dawkins, 13, at New Dominion School in Manassas. Teachers randomly assigned students to different tables.