A national school milk program intended to improve children's health has soured again.

On Friday morning, 251 children at Yuhe Town Central Primary School in Yulin, Shaanxi province, fell ill after drinking school milk produced by the Mengniu Dairy Group. They were sent to hospital for treatment, and were all discharged by the following day.

Test results released on Tuesday afternoon said the milk met China's national standards for food safety, and no pathogens were detected in the milk or the students' vomit and feces. They did not say why 251 students suffered from the discomforts collectively.

The tests were conducted by Yulin's Center for Disease Control and Prevention and the results were announced by the city's food safety committee.

On Tuesday evening, Mengniu said on its website that it would work with the school and the local health department to track the students' health. But public confidence in China's developing school milk program suffered another blow.

Dairy enterprises are impaired by suspected and real poisoning incidents and low profits. Schools stay out of the program because they do not want to be implicated in safety scandals. And the country has no unified management or supervision of the program it started in 2000.

Milk producers and local watchdogs are the first to be blamed for bad quality control, but experts also have doubts about the guiding theory behind the program.