BEIJING — Chinese families were in an uproar on Monday after a report in the state news media revealed that nearly 500 students at a school in eastern China had developed illnesses, a few as severe as leukemia, possibly because of pollution at a nearby field.

Students and parents at the Changzhou Foreign Languages School had complained since December about pollution in the area, after dozens of children came down with rashes and nosebleeds, and a foul stench surrounded the school. But local officials dismissed their concerns, saying that the air, soil and groundwater met national standards.

On Sunday, the government’s powerful national broadcaster, China Central Television, aired a scathing report documenting the illnesses and finding that toxins in the soil and water far exceeded national limits. The broadcast of the report suggested that China’s leaders were taking a more aggressive stance toward chemical companies at a time when public anger over environmental pollution is mounting, especially in the aftermath of a chemical disaster last year that killed 165 people in the port city of Tianjin.

But the episode also underscored the large gaps that exist in China’s oversight of hazardous materials. While China has made strides in publicizing air and groundwater pollution data in recent years, it still does not provide data on local soil pollution or require companies to publicly list which substances they discharge as waste, a departure from international standards.