An investigation has been launched into why dangerous corrosive acid was used in swimming pool changing rooms after schoolchildren and staff - including a pregnant woman - fell ill.
The six and seven-year-old pupils were not evacuated from the building despite the workmen who were using the chemicals leaving because they were overwhelmed by the fumes.
The students, from St Michael's School in Helston, Cornwall, were participating in a swimming lesson at their local sports centre, owned by Carrick Leisure Ltd, last week. On their arrival back at school, some students complained of nausea.
One boy was taken to the doctor after complaining of a tight chest, the West Briton reported, while a pregnant teacher and a number of staff attended a nearby hospital.
Headteacher Shaun Perfect said he was "disappointed" the cleaning products were used while the children were at the centre.
"My understanding is that contractors carrying out maintenance of the drains used some strong drain cleaner which contained sulphuric acid," he said.
The Health Protection Agency confirmed the drain cleaner contained sulphuric acid but said it was "unlikely" the children had been harmed.
A spokeswoman for Cornwall Council said an investigation had been launched and health and safety officers were currently trying to establish why the cleaner had been used.
A statement from the sports centre confirmed a "small amount of chemical was used by the sub-contractor".
"The fumes affected the workmen who had to leave the building. Although no other people were in the actual changing room a school class were assembling in the adjoining corridor and some students reported being aware of the fumes.
"In response the centre staff vacated the areas and made arrangements for the school class to change in another area. Action was also taken to ventilate the changing rooms."