Cladding from four tower blocks in Swansea and three in Newport have now failed fire safety checks. It comes in the wake of the Grenfell Tower tragedy in London, which killed at least 80 people.
Rhyl High School will reopen on Monday 10 July after closing for two days due to concerns over cladding.
A decision was taken to close the school last Wednesday after it emerged the cladding used as part of the new building came from the same manufacturer that provided the cladding for Grenfell Tower.
In a joint statement, Denbighshire County Council and Rhyl High School said safety experts have carried out a rigorous investigation and found the building to be fully compliant, with no additional risk of fire.
They thanked pupils and parents for their patience, stressing that pupil safety is "paramount".
Cladding tests on three tower blocks in Newport have revealed further concerns over the safety of the buildings.
Rhyl High School will be closed for the rest of the week, while cladding used on its new building is checked.
Swansea Council says cladding systems on its four high-rise block have passed 'whole-system' safety checks.