A teenage girl had to have most of her fingers and thumbs amputated because of severe burns caused by mixing plaster of Paris by hand during an art class.
Art teachers are being warned about the potential danger
The 16-year-old girl is said to have suffered third degree burns as the plaster set.
It happened in January at Giles School and Sixth Form Centre in Old Leake, Lincolnshire, a visual arts specialist.
The Health and Safety Executive is investigating and safety warnings are being circulated to other schools.
The Giles School's head teacher, Chris Walls, said in a statement: "I can confirm that a serious accident did occur in our art department at the end of January and that the Health and Safety Executive are involved.
"They are in the initial stages of their enquiry into this accident and we do not wish to make any further comment until this has been completed."
The executive told the BBC News website that the girl had lost her thumbs and most of her fingers.
The warning is now circulating in safety bulletins distributed to schools by local authorities.
It was sent out by the school science advisory service, Cleapss.
"In the incident a girl was mixing plaster of Paris with water by hand, intending to make a plaster cast of her hand," it said.
It said there had been a similar incident a few years ago, in which a pupil had lost two fingers.
"Plaster of Paris is of course used for setting fractured bones but in such cases it is never put in direct contact with the skin.
"In addition, it is used in relatively thin layers so the heat can dissipate. In bulk, the temperature can reach 60°C or so."
Other experts say plaster and similar products such as stucco and concrete do get extremely hot as they set.
One industry website warns: "When you pour a plaster statue or mix a large batch of plaster a chemical reaction takes place during the setting process and the cast plaster heats up as it hardens.
"The larger the volume you cast, the more it will heat up. Temperatures may reach 150° deep within the mass of a large casting.
"Do not encase any of your body parts or anyone else's in plaster."