The teenager was attempting to make a sculpture of her own hands during a lesson in January 2007 when the accident happened, Boston Magistrates' Court in Lincolnshire heard.
The plaster set around her hands and neither staff nor paramedics could get it off during the lesson at Giles School, in Boston.
The court was told that temperatures up to 60C can be generated in large quantities of plaster and the girl, who was not named in court, suffered terrible burns.
After a series of 12 operations, she was left with no fingers on one hand and just two on the other.
The school's governing body admitted breaching health and safety regulations and also failing to report the incident to the Health and Safety Executive (HSE).
The court was told the HSE was never informed by the school about what had happened and only found out six weeks after the accident from the girl's plastic surgeon.
The school was fined a total of 16,500 pounds and ordered to pay 2,500 pounds in legal costs, the Press Association reported.
The girl's lawyer Stephen Hill said outside court the injuries his client, now 18, had suffered were "truly horrific" and she also had severe scars all over her body where the plastic surgeons had taken skin for grafts.
However, Hill said she was now doing "remarkably well."
"She is a very determined, self sufficient character but she is now only left with one forefinger and an index finger," he said.
"Teachers have a responsibility to ensure when children are using chemicals they are used safely."