The Ministry of Environmental Protection started an investigation on Sunday into what caused about 500 students at Changzhou Foreign School in Jiangsu province to come down with a range of pollution-related illnesses.
The school had relocated in September to a facility built on a toxic site.
The spate of illnesses that flared since then include leukemia, dermatitis, eczema, bronchitis and blood abnormalities.
According to the school, 641 students were examined recently by doctors, and 493 were diagnosed with various diseases and conditions.
The local government confirmed the new campus was built on a brownfield site that had housed three chemical factories.
The plants had produced highly toxic chemicals including carbofuran and methomyl.
People who formerly worked in the factories said they had buried toxic chemicals near the plants to save time and money.
They also spoke of discharging untreated wastewater into rivers.
An environmental report showed that soil and nearby groundwater contained organic pollutants including chlorobenzene and carbon tetrachloride. Heavy-metal pollutants such as mercury, lead and cadmium were also found.
The concentrations of chlorobenzene in the groundwater and soil greatly exceeded normal levels, according to the report.
The amount of chlorobenzene in the groundwater was 94,799 times normal. It was 78,899 times higher in the soil.
Pan Xiaochuan, a professor at the public health school under Peking University, was quoted by China Central Television as saying that the high number of students being diagnosed with diseases and conditions in such a short space of time should be connected with the heavy pollution levels.
Professors specialising in environmental issues told CCTV the environmental assessment report that justified the construction of the new campus did not look for pesticides.
And they said that builders had used heavily polluted groundwater during the construction process.