By SAM DILLON

SEOUL, South Korea - It is 10:30 p.m. and students at the elite Daewon prep school here are cramming in a study hall that ends a 15-hour school day. A window is propped open so the evening chill can keep them awake. One teenager studies standing at his desk to keep from dozing.
Kim Hyun-kyung, who has accumulated nearly perfect scores on her college entrance exams, is preparing for physics, chemistry and history exams.
“I can’t let myself waste even a second, said Ms. Kim, who dreams of attending Harvard University, Yale University or another prestigious American college. And she has a good chance. This spring, as in previous years, all but a few of the 133 graduates from Daewon Foreign Language High School who applied to selective American universities won admission.
“Going to U.S. universities has become like a huge fad in Korean society, and the Ivy League names - Harvard, Yale, Princeton - have really struck a nerve, said Victoria Kim, who attended Daewon and graduated from Harvard last June.
Despite the rigorous regimen, some students seem to enjoy the challenge. A recent Daewon graduate, Kim Soo-yeon, 19, has been accepted by Princeton University in New Jersey. Daewon parents tend to be wealthy doctors, lawyers or university professors. Ms. Kim’s father is a top official in the Korean Olympic Committee.
Ms. Kim developed fierce study habits early, watching her mother scold her older sister for receiving any score less than 100 on tests. Even a 98 or a 99 brought a scolding. “Most Korean mothers want their children to get 100 on all the tests in all the subjects, Ms. Kim’s mother said.
Ms. Kim’s highest aspiration was to attend a top Korean university, until she read a book by a Korean student at Harvard about American universities. Immediately she put up a sign in her bedroom: “I’m going to an Ivy League!
Even while at Daewon, Ms. Kim, like thousands of Korean students, took weekend classes in English, physics and other subjects at private academies, raising her entrance exam scores by hundreds of points. “I just love to do well on the tests, she said.
But other pleasures are frowned on. Both schools suppress teenage romance as a waste of time.
“What are you doing holding hands? a Daewon administrator scolded an adolescent couple recently, according to his aides. “You should be studying!
Students do not seem to complain. Park Yeshong, one of Kim Hyun-kyung’s classmates, said attractions tended to fade during hundreds of hours of close-quarters study. “We know each other too well to fall in love, she said.
Daewon has one major Korean rival, the Minjok Leadership Academy, three hours’ drive east of Seoul, which also has a spectacular record of admission to Ivy League colleges.
How do they do it- Their formula is relatively simple. They take South Korea’s top-scoring middle school students, put those who aspire to an American university in English-language classes, taught by Korean and highly paid American and other foreign teachers, emphasize composition and other skills crucial to success on the entrance exams and college admissions essays, and - especially this -urge them on to unceasing study.
Both schools seem to be rethinking their grueling regimen, at least a bit. Minjok, a boarding school, has turned off dormitory surveillance cameras previously used to ensure that students did not doze in latenight study sessions. Daewon is ending its school day earlier for freshmen.
Both schools reserve admission for highly motivated students. “Even my worst students are great,’’ said Joseph Foster, a Williams College graduate who teaches writing at Daewon. “They’re professionals; if I teach them, they’ll learn it. I get e-mails at 2 a.m. I’ll respond and go to bed. When I get up, I’ll find a follow-up question mailed at 5 a.m.
South Korea is not the only country sending more students to the United States, but it seems to be a special case. Some 103,000 Korean students study at American schools of all levels, more than from any other country, according to American government statistics. In higher education, only India and China, with populations more than 20 times that of South Korea’s, send more students.
“Preparing to get to the best American universities has become something of a national obsession in Korea, said Alexander Vershbow, the American ambassador to South Korea.