LOS ANGELES — Zhao Yan Feng finally lost his cool minutes before the bell sounded, signaling the end of fourth period.

For nearly two hours, his classroom had teetered on anarchy. Students chatted on their cell phones. They put their feet up on desks. Some had their heads down, sleeping. A clique of girls debated loudly where best to shop for jeans.

"I need your cooperation," Zhao pleaded in a clumsy Chinese accent. "If you don't want to learn this language or be in my class, just don't interfere with others learning. I'm just a guest teacher."

It had been three weeks since Zhao (pronounced Jow), 27, left northern China in a program that sent dozens of Chinese teachers to U.S. schools.

His two-year assignment: teach Mandarin at Dorsey High in South Los Angeles, where test scores are well below state and national average, two-thirds of the students live near the poverty line and most have had scant exposure to Chinese culture.

"Why do I do this?" he said to the students, who were silent for the first time. "Because I want to be your friend."

Two girls in the back of the classroom giggled at the remark. The bell rang, and the teens charged out the room — except for a boy who was still asleep. Zhao tapped him on the shoulder and told him to leave.

It was the end to another humiliating day.

"Two years," Zhao said. "Sometimes I don't know how I'll do it."

Zhao had been told by other Chinese teachers that American high schools can be tough. When he arrived, he saw police stationed in squad cars outside the campus. He assumed there had been an incident. He soon learned the police are there all the time.

Zhao seemed to be the only Asian on a campus of mostly black and Hispanic students.

In China, teachers traditionally command unquestioned authority. At Dorsey, the few good students were being overshadowed by those who walked around the classroom to talk to friends, sent text messages and defied Zhao's orders to pay attention. He could not understand why his efforts — traveling all the way from China to teach — meant nothing to so many.

It didn't help that the students could not understand Zhao's stilted English at times, and that he rarely offered encouragement. In China, a simple "hao," meaning "good," is often the extent of their praise.

"You can't put these characters together, it's wrong," Zhao sternly told a girl during a writing exercise. As Zhao walked away, the student carried on, unsure how to do it correctly.

After varied attempts to connect with the students, he held a quiz — 15 true-or-false questions, a translation exercise and a stab at writing a few basic words in Chinese characters. This was the first time Zhao would see if his students had learned anything.

As Zhao dropped the tests on their desks, a girl picked hers up, examined it and loudly blurted an expletive. For the next 45 minutes, some in the class attempted to answer such questions as: "Chinese culture was the cradle of Japanese, Korean and some Southeast Asian cultures — true or false?" Others looked as if they were sleeping with their eyes open.

But something changed the moment Zhao started explaining the answers. A competitive spirit emerged among some.

When they got a question correct, they cheered. When they got another one right, they moved their shoulders in a dance.

"I knew that one. I'm a genius. That was easy," said a playful Alena Monet Cox, a 12th-grader who straddled the line between those who misbehaved and those who paid attention.

Her enthusiasm set the tone. When she participated, so did the clique around her. On this day, Zhao was impressed. Monet, as she chose to be called, had even remembered how to write her name in Chinese. She had signed her quiz that way.