A pupil suffered 40% burns after lighting candles on a birthday cake at a school, the fire service said.
The girl - thought to be aged 17 - was taken to hospital following the incident at St Bernard's High School, in Southend on Friday.
Martyn Hodder, of Essex Fire Service, said a candle ignited the girl's fancy dress costume at about 12.40pm.
"The pupil who was hurt was dressed as a snowman and covered head to foot in cotton wool," he said.
"She was lighting the candles on a cake and it appears that a flame came in contact with the suit and the fire lit and spread very rapidly."
Part of the school, which was holding a fancy dress day, was smoke-logged but the fire did not spread, Mr Hodder said.
He added: "The school did an excellent job in evacuating and dealing with pupils in this distressing situation, especially the pupils who had witnessed such a distressing incident."
Mr Hodder warned schools not to use cotton wool as part of any fancy dress or nativity costume if there was a chance it could come into contact with a naked flame.
