High levels of formaldehyde were discovered at Flash Ley Community Primary School and Nursery in Stafford after pupils fell ill last October.
Students have been forced to attend lessons elsewhere.
Staffordshire County Council said it could not provide a date for their return but it was doing all it could to reopen the site as soon as possible.
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The authority said an investigation concluded work to floor ducts last summer led to high levels of the chemical.
It said air quality in "some parts" of the school was "yet to reach safe levels" and work there would start "as soon as air testing revealed it was safe to do so".
Cabinet member for learning and skills, Ben Adams, said he wanted to thank parents and staff "for their continued patience and support".
"We want all of the pupils and staff to return to the school as soon as possible, but the safety of the whole school community will always remain our top priority," he added.

Carl Marsh, 46, of Knutsford, Cheshire suffered fatal injuries at Black Knights Parachute Centre in Cockerham, near Lancaster on Saturday.
He was pronounced dead at the scene.
The British Parachute Association (BPA) said the jump Mr Marsh was executing could only have been attempted by an experienced parachutist like Mr Marsh.
Tony Butler, the BPA's chief operating officer, is leading the probe into the death and said it was his "top priority".
He estimated it would take about a month to complete his investigation and he would send his findings to the Civil Aviation Authority, the police and the coroner as well as the BPA's training committee.
Lancashire Police, which is supporting the BPA's investigation, said there were not thought to be any suspicious circumstances surrounding the death.
Mr Marsh's relatives said he was a "much-loved" family man with an "enormous heart".

Most of those infected were foreign construction workers, and all lived or worked within the same region of the country, said the health ministry.
None were known to have travelled to Zika-affected areas recently. Thirty-four people have fully recovered but seven are still in hospital.
Zika generally has mild effects but has been linked to severe birth defects.
It is suspected of causing a condition called microcephaly, a deformation in which babies are born with small heads.
Zika, which is spread by mosquitoes but can also be sexually transmitted, was declared a global public health emergency by the World Health Organization in February.
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Singapore declared its first imported Zika case in May, a 48-year-old local who had travelled to Brazil, where the outbreak started.
On Saturday, it announced that a 37-year-old Malaysian woman had contracted the virus locally. She is the only confirmed female case so far.
Thirty-six of the 41 cases announced on Sunday were foreign workers who worked in the same construction site.
Work at the site has been halted after housekeeping was found to be "unsatisfactory with potential breeding habitats favourable to mosquito breeding".
The ministry said in a statement that it could not "rule out further community transmission, since more of those tested positive also live or work in other parts of Singapore".
"We expect to identify more positive cases," it added.
Repair work has not begun in parts of a school a year after it closed as the air quality is still not safe.
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A parachutist who died performing a jump in Lancashire was attempting a "complicated manoeuvre", the man investigating his death has said.
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Singapore has confirmed 41 cases of the Zika virus which were locally transmitted.