Blue tap water at two schools has been found to contain three times the legal level of copper.
Pupils and teachers at Buchanan High and St Ambrose High in Coatbridge, North Lanarkshire, were told not to drink tap water in December, three years after the first warnings that it was running blue.
Now the results of tests show the amount of copper in the water was more than three times the permitted levels in the home economics room and nearly double in the school kitchen.
The tests also revealed water in the staff room and the school kitchen failed to meet regulated standards because of the presence of coliforms – bacteria which indicate faecal contamination.
The schools share a campus, which was built on a former landfill site for industrial waste.
However, the contamination may come from the copper pipework, which is being replaced with plastic.
Teachers have raised concerns over the blue water issue affecting their health, with figures showing Buchanan High has the highest rate for teacher sickness absence in North Lanarkshire.
But legitimate questions do remain over what is causing this in the first place.
Microbiologist Professor Hugh Pennington said: “There shouldn’t be any coliforms in drinking water, there should be zero, so there is something wrong somewhere.
“Coliforms themselves are a harmless bug, but they indicate faecal contamination.”