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."
US player Serena Williams plays a forehand shot during her second round women's singles match against Hungary's Melinda Czink on day four of the 2012 Wimbledon Championships tennis tournament at the All England Tennis Club in Wimbledon, southwest London, on June 28, 2012. (Photo credit: MIGUEL MEDINA/AFP/GettyImages)
A women protests against the Obama administrations health care plan during a protest in front of the U.S. Supreme Court, on June 28, 2012 in Washington, DC. Today the high court is expected to rule on the constitutionality of the sweeping health care law championed by President Barack Obama. (Photo credit: Mark Wilson/Getty Images)
Rowers in traditional blazers watch the action at Henley Royal Regatta on June 28, 2012 in Henley-on-Thames, England. Held since 1839, this social and sporting event is a highlight of the season and takes place along the one mile 550 yards of Henley Reach. (Photo credit: Chris Jackson/Getty Images)
Members of the public enjoy the atmosphere at Henley Royal Regatta on June 28, 2012 in Henley-on-Thames, England. Held since 1839, this social and sporting event is a highlight of the season and takes place along the one mile 550 yards of Henley Reach. (Photo credit: Chris Jackson/Getty Images)
Belly dancers perform in front of the U.S. Supreme Court on June 28, 2012 in Washington, DC. Today the high court is expected to rule on the constitutionality of the sweeping health care law championed by President Barack Obama. (Photo credit: Mark Wilson/Getty Images)
3D artwork of a bowl of cereal is laid out in Canary Wharf in London, to promote Jordans new granola and to celebrate the cereal brand's founder, Bill Jordan, bringing granola to the UK from the US 40 years ago. (Photo credit: Matt Crossick/PA Wire)
A Bangaldeshi man sleeps with his child at a shelter after a flood and a landslide in Bandarban, Bangladesh, Thursday, June 28, 2012. Searchers pulled out 15 bodies Thursday that had buried under mud and debris after landslides and floods caused by monsoon rains in southern Bangladesh, raising the toll to 106 deaths this week. (Photo credit: AP Photo/Anurup Titu)
French President Francois Hollande arrives for an EU Summit in Brussels on Thursday, June 28, 2012. European leaders gathering Thursday in Brussels are set to sign off on a series of measures to boost economic growth but expectations of a breakthrough on the pooling of debt have fallen by the wayside. (Photo credit: AP Photo/Geert Vanden Wijngaert)
Protestors hold masks of EU leaders as they demonstrate outside an EU Summit in Brussels on Thursday, June 28, 2012. The group protested to encourage EU leaders to not lose their priorities for development cooperation. EU leaders will meet for a crucial two-day EU Summit amid growing concerns over the financial health of Greece, Spain and Italy. (Photo credit: AP Photo/Virginia Mayo)
Torchbearer Laura Graves stands in front of The Major Oak in Sherwood Forest whilst holding the Olympic Flame, joined by 21st century Robin Hood Ade Andrews and Sylvia Robson (right). (Photo credit: PA)
Schoolchildren wave flags as they wait for the first Torchbearer of the Day outside Lincoln Cathedral during Day 41 of the London 2012 Olympic Torch Relay. (Photo credit: PA)
Torchbearer Dorothy Fraser holds the Olympic Flame outside Lincoln Castle at the beginning of Day 41 of the London 2012 Olympic Torch Relay. (Photo credit: PA)