Three schools will remain closed after bacteria that can cause Legionnaires' disease was found in pipes.
Brynmawr Foundation School, Blaen-y-Cwm Primary School, in Brynmawr, and Ystruth Primary School, in Blaina, will stay shut until at least 13 July.
Blaenau Gwent Council previously said there was a "localised contamination" of legionella.
The schools' water system is being chlorinated and will be retested to ensure it is safe.
A council spokesman said the contamination was "low level".
A letter sent to parents last week from Brynmawr Foundation School's head teacher Gerard McNamara said the discovery of legionella would "require a full-system chlorination."
Legionella - which causes Legionnaires' disease - occurs in natural water sources such as rivers and lakes but usually in low amounts.
In certain conditions, such as when water is static for a long time, bacteria can grow and become hazardous.
Joanne Collins, executive member for education said: "This is disappointing for the pupils who planned to return to school and their families, but the safety and wellbeing of all our pupils and school staff is always the top priority."
She told the Local Democracy Reporting Service health teams were working to resolve the matter as "quickly as possible."
Labour councillor Hedley McCarthy said: "An urgent independent inquiry should be set up by Welsh Government to give transparency as to how this could have been allowed to happen."
Public Health Wales said it was currently "not aware of any risk to the population."
Welsh Water said internal pipework problems posed no risk to water supplies.
Tredegar Comprehensive and Willowtown Primary School, in Ebbw Vale, also did not reopen as planned on Monday but it is hoped they will be able to this week.