Karla Caraballo left her home in the Dominican Republic clinging to a word of advice from her grandfather: If someone speaks to you in English, just shake your head. So when a flight attendant asked her that night if she wanted something to eat, the shy teenager said no -- and arrived in America so hungry it hurt.

Caraballo laughs when she tells the story now. Only three years later, she is the valedictorian of one of the District's best high schools, a student of Shakespeare on her way to Emory University with a full scholarship. In her free time, she reads medical school brochures.

Ask her how she managed to learn English so quickly while keeping up with -- and pulling ahead of -- her American peers, and she smiles. "People ask me that all the time, and I really don't know how to answer."

Caraballo's success is echoed by scores of immigrant students who graduated with honors last month, just a few years after they arrived in the Washington area knowing little or no English.

Some were named valedictorians, including My-Le Nguyen, whose first days in a Rockville middle school were so difficult she often hid in a stairwell and cried over her Vietnamese-English dictionary. Some, such as Jasenka Drincic, had to overcome the trauma of war. The young Bosnian refugee lived in so many places before settling in Fairfax County that English was actually her fourth language. Many will be going to college, among them Wubshet Ayalew, an Ethiopian student in Prince George's County who pumps gas on weekends but dreams of becoming an astronaut.

Together, their stories offer clues to the mysteries of language and learning. Although educators have spent years studying how best to teach the large numbers of children entering elementary school without English, much less is known about how to help immigrant teenagers going straight to secondary schools. There, they must catch up with peers who have been reading and writing in English much longer, and they must master more complex academic material -- while the clock ticks toward graduation.

`Challenge Yourself'

Caraballo and the others are products of school systems that, for practical and philosophical reasons, are developing a new approach to teaching "language-minority" teenagers. It's not bilingual education, which is rare in secondary schools, nor is it the sink-or-swim system that greeted young immigrants a century ago. Yet the emphasis is unmistakably on teaching everything in English.

"I'd sit there, and I didn't know what they were talking about," Caraballo, 18, said of her first few months at the District's Bell Multicultural High School. "I spent more time in the dictionary than reading."

She and her brother, Hugo, 19, who immigrated a few months after his sister, took English as a Second Language classes. Their other classes at Bell were taught by teachers trained to work with students with limited English. The approach, known as "sheltered content" instruction, is based on the theory that students learn English best when it is taught in the context of academic material they have to learn anyway.

Most Washington area high schools use the strategy, though the number of "sheltered" courses varies. In these classes, teachers try to identify grammar patterns and words that might give students trouble. They also use diagrams, pictures, maps, labs -- anything to get material across without relying on language.

"Sometimes, when you wouldn't have a clue, they'd explain it in a simple way, and you could understand," Karla Caraballo said. "In history class, every time somebody didn't understand a word, we'd define it and put it on the board."

The unique environment at Bell, where four in five students are foreign-born, also helped her succeed. "Nobody laughs at you, because everyone has the same problem," she said. "Everyone's trying to learn English."

Both Hugo and Karla spent long hours studying, waking each other at 4 a.m. to hit the books before school. They also benefited from the support of their parents, who immigrated five years earlier. Their father works at a golf course and cleans offices, their mother is a Spanish teacher and day-care provider, and both speak less English than their children.

Despite the difficulties of English immersion, Hugo and Karla said classes taught in Spanish only would have slowed them down. "If you take Spanish classes, you'd never learn English," she said. "You've got to challenge yourself and try to understand. What you don't, you ask somebody or look it up."

Maria Tukeva, Bell's principal, said forcing students to take classes in English helps prepare them for college -- where everything will be in English. "If you don't give them enough exposure in English, they're going to be at a disadvantage in taking SATs, writing application essays and when they're in college," she said. "They've got to learn to adjust, and they only have a few years to do it."

Tukeva said the approach works best for students with strong prior schooling in their native languages, and the Caraballo siblings said their Spanish education in the Dominican schools made it easier for them to learn English. "You know grammar; you know how to structure a sentence," Hugo said. "When a teacher talked about adjectives, subjects, verbs, I knew what that was."

Researchers agree that schooling in a student's native language is the single best predictor of academic success for immigrant teenagers -- more than family income, previous English study or the type of support schools provide.

The prospects for newcomers who arrive with little education are much less certain. That group includes students with schooling interrupted by war and others who came from rural areas where there were no schools. Many can't read in their native languages or do so only at an elementary school level. It's difficult to say how many there are, because some never enroll in school here.

In a recent Washington Post poll of the region's public high school teachers, 11 percent of those surveyed said that in at least half their classes, they had encountered students who could not read or speak English well enough to keep up with the class. Forty-four percent of those polled reported a similar situation in a few of their classes, and 41 percent said it was not a serious problem in any of their classes.

Several area school systems are experimenting with new programs for those "under-schooled" immigrants, including evening and summer classes; intensive courses in basic math, grammar and study skills; and literacy classes in their native languages. But it's unclear whether any of them work. Community activists say the public schools continue to fail those children.

Karla and Hugo Caraballo can name classmates who dropped out, girls who became pregnant and boys who joined gangs or went to work to support themselves or their families.

"There's a guy who came to school the same day I did, and afterwards he just quit," said Hugo, who will attend Colby College. "I saw him on the street one day. . . . It was painful, because we both started school together with the same hopes."

`It Was Scary'

If the Caraballos felt at home among their diverse classmates at Bell, My-Le Nguyen encountered a more trying -- and more typical -- environment at her middle school in Rockville.

She was 14 and in the eighth grade. After 11 years of waiting, her family finally had received visas to immigrate to the United States. "I started school one week after I arrived," she said. "It was scary. . . . Everything was so different from the school in Vietnam."

Every day that first week, Nguyen skipped lunch, hid in a stairwell and cried. She struggled to learn to use a locker, got lost in the hallways and worried constantly about missing the school bus. Classmates teased her. And she wondered whether she would ever learn enough English to graduate.

There were no classes in Vietnamese. Students in the Washington area speak more than 130 languages, and such diversity has made it difficult for school systems to offer native-language instruction even in Spanish, the most common language. There also is political resistance to bilingual programs in many communities.

Nguyen was placed in an ESL class and in other classes taught in English, including a U.S. history class that was "sheltered" for English learners. In math and science, she sat with regular students.

"In math, it was just numbers, so that was easy. But in science class, I had to look up everything in the dictionary," she said. "I could understand maybe 20 percent of what was going on."

But Nguyen worked closely with her ESL teacher, who helped her after school once a week. She continued with ESL over the summer and the next year at Rockville High School. By her sophomore year, she was in a regular English class. By her senior year, she was in Advanced Placement English.

Now 18 and on her way to the University of Maryland-Baltimore County, Nguyen attributes her success to patient teachers who spent more time with her. Her teachers say it also helped that she studied every day after school until 10:30 p.m., usually doing homework while eating dinner.

Nguyen's education in Vietnam was critical not only because it provided a basic knowledge of grammar and math but also because it gave her strong study skills.

"Over there, when you learn something, you would have to memorize the whole lecture and the teacher would call on you. That's just a lot different from the American way," she said. "But it was like training, and it helped."

`I Was So Frustrated'

Jasenka Drincic had just finished the fourth grade when she had to leave her home in Sarajevo. Shortly after a sniper's bullet grazed her little brother's leg, her parents decided it would be best if the children left.

"It was supposed to be like a vacation for two weeks. Nobody in Sarajevo wanted a war, and nobody thought it could happen," she said. "For our safety, we left with our mother for Belgrade."

But the vacation turned into an odyssey. During the next two years, she moved from Belgrade to Macedonia to Belgrade to Montenegro to Belgrade to a mountain village near Bosnia and back to Belgrade again. Only occasionally would she hear from her father, who was trapped in Sarajevo.

Drincic was able to attend school regularly again in the seventh grade, when a refugee agency moved the family to a town near Barcelona. She took English and Spanish, but her classes were taught in Catalan.

Then, in 1995, as Spain began pressuring the refugees to leave, the family moved to Fairfax County. Drincic had to start over again with another language.

"I was so frustrated. . . . I thought I would never learn English," she said. She survived initially because Latino classmates would translate English to Spanish for her.

Sitting in class, she also would worry about her father, who didn't make it to America until later. "Sometimes we didn't know if he was alive or not, so it was very hard to concentrate," she said.

Drincic took ESL in middle school, summer school and then at Mount Vernon High School. During her freshman year, she took two classes -- world civilization and biology -- in which an ESL teacher helped the regular teacher.

Researchers say such "team teaching" can be very effective for students with limited English. But it is costly, and a shortage of trained ESL teachers means it isn't very common.

Drincic, now 17 and headed for James Madison University, said it made a big difference. "Without the extra teacher," she said, "I would have been lost."

`I Have a Chance'

Wubshet Ayalew had already studied English for a few years in Addis Ababa when he came to America three years ago. But like many young immigrants, he was surprised by how bad his English was.

"I guess we learned the grammar and everything, but we never spoke it," he said. "We'd write in English, but we when we talked, we'd change to Amharic."

Still, the grounding helped him move out of the ESL program quickly. After one ESL class and one "sheltered" government class for students with limited English, he was taking courses with everybody else at Northwestern High in Hyattsville. He worked weekends at a gas station, studying vocabulary lists behind the register. He never went out with friends, because he had none.

"But that's okay," he said. "I like to be alone so I can concentrate on my education."

Ayalew cruised through most of his classes, but sometimes his weaker English skills haunted him.

"When I was taking AP physics, I couldn't understand some of the problems," he said. "I could do the math. That was easy. But you had to understand the words, and some of them I had never seen before."

Sometimes, he asked the teacher to explain. Other times, he was too embarrassed. "I didn't ask him, because the other students didn't ask," he said.

Students who finish ESL often struggle several more years before they truly catch up to their peers, researchers say. But Ayalew said more ESL classes wouldn't have helped him: "At some point, I have to do it by myself."

He graduated this year near the top of his class, with nearly all A's. The 18-year-old plans to attend the University of Maryland-Eastern Shore to study aviation and hopes to become an astronaut.

"In Ethiopia, I'd have no chance," he said. "Here, I have a chance, so I'm going to try."

CAPTION: Hugo Caraballo, left, and his valedictorian sister, Karla, center, are hugged by mother Julia Monk, father Hugo Sr. and brother Victor.

CAPTION: Jasenka Drincic, center, a refugee from Bosnia, studies with Yin Wu, of Fairfax, left, and Susan Desta, of Richmond.

CAPTION: Dominican immigrants Julia Monk and her daughter, Karla Caraballo, fix lunch at their home in Washington. Karla won a full scholarship to Emory University.