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.
Repair work has not begun in parts of a school a year after it closed as the air quality is still not safe.