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 upright at his desk to keep from dozing.

Kim Hyun-kyung, who has accumulated nearly perfect scores on her SATs, is multitasking to prepare for physics, chemistry and history exams.

“I can’t let myself waste even a second,” said Ms. Kim, who dreams of attending Harvard, Yale or another brand-name American college. And she has a good shot. 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.

It is a success rate that American parents may well envy, especially now, as many students are swallowing rejection from favorite universities at the close of an insanely selective college application season.