More than 12,000 students in Anshan, Liaoning province, were forced to retake two tests in the senior high school entrance exam on Monday, after a massive cheating scandal was uncovered.
Song Guochun, vice-director of the city's bureau of education, said a report was received on June 28 that test questions on two subjects in the exam had been leaked.
Some students said the test papers were "identical" to ones they had bought a few days earlier.
After the first test - Chinese - on the morning of June 28, the classrooms were packed with students with copies of the science test paper. The test was to be held in the afternoon.
Sources said each copy of the test papers were sold for between 2,500 yuan ($365) and 5,000 yuan.
On the afternoon of June 29, when the students had completed tests on two other subjects, English and mathematics, the authorities declared the Chinese and science tests invalid and that all students would have to retake them the following day.
Song said the cheating took place in four districts of the city - Tiexi, Tiedong, Lishan and Qianshan - affecting more than 12,000 students.
"The case is still under investigation," he said.
"It's still too early to say who is responsible."
The Xinhua News Agency quoted a spokesman with the city's bureau of public security as saying police launched an investigation immediately after the cheating was reported.
Yan Haitao, director of the city's education bureau, said at a meeting on June 24 that the exam would be "perfectly safe from cheating".