HANOI, Vietnam — The thugs came after dark, as Do Viet Khoa and his family were getting ready for bed.

He says they punched him, kicked him, stole his camera and terrified his wife and children.

Khoa, a high school math and geography teacher, thinks the message was clear: Stop blowing the whistle on school corruption — or else.

For several years, Khoa has been fighting the petty bribery and cheating that plagues schools across Vietnam, where poorly paid teachers and administrators squeeze money out of even poorer parents.

Vietnam's leaders approved a sweeping anti-corruption law in 2005, but implementation is uneven. The country still ranks poorly on global corruption surveys, and for ordinary Vietnamese, who treasure education, school corruption is perhaps the most infuriating of all.

Few dare to fight it, for fear of retaliation.

A slight, ordinary-looking man from a farming village, 40-year-old Khoa made a dramatic entrance onto the national scene two years ago. He videotaped students cheating on their high school graduation exams while their teachers watched and did nothing. State-owned TV stations played the tape repeatedly.

With TV cameras in tow, Vietnam's education minister went to Khoa's house to hand him a certificate praising his courage. Khoa appeared on Vietnam's version of the Larry King show. The principal of the Van Tao High School, where Khoa has taught since 2000, was transferred.

But back in his farming village of Van Hoa, about 15 miles outside Hanoi, Khoa got anything but a hero's welcome.

Teachers and administrators resented the unflattering spotlight. Even among parents and students, who stood to gain most from Khoa's efforts, few came to his defense.

All the parents wanted was to get their children through school and into jobs, even if they had to cheat to pass their exams, Khoa said.

"The entire community has shunned me," Khoa said. "They harass me on the phone, they send me letters. They say I put my thirst for fame ahead of their children's welfare. Some of them even threatened to kill me."

One of Khoa's biggest complaints is the "extra classes" implemented at his school and others across the country, in which regular school teachers tutor students for money.

"If they don't go, the teachers give them bad grades," said Khoa.

One man defending the teacher is Vu Van Thuc, whose son goes to the school. "He is raising his voice against these absurd requirements imposed by the school," he said.

"He is really brave," said Giang Xuan Dung, a math teacher. "I admire him for his courage and patience."

Other schools have offered to hire Khoa.

"I thought we should support him," said Van Nhu Cuong, a Hanoi headmaster who tried to hire him. "We really need people who dare to speak out."