A water sample taken from Fanling Lutheran Secondary School has been found to exceed the World Health Organisation lead limit.
The Education Bureau said the sample comes from the wall-mounted water dispenser in the school's tuck shop.
It was found to contain 11 micrograms of lead per litre, exceeding the WHO standard of 10.
There is one lactating mother among the school staff and the Department of Health will arrange blood test for her.
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