The fumes were so strong after the fire broke out at Merrol Hyde Magnet School in Hendersonville, Tennessee, on Wednesday that all pupils and staff had to be evacuated.
Eight students and a teacher were taken to hospital with chemical burns or splash injuries which were not life-threatening.
Fire chief Scotty Bush said the incident was an accident and that the students were gathered around as the teacher did a demonstration that involved mixing chemicals.
There was a sudden flash fire lasting about five seconds, but because there was no smoke fire alarms were not set off.
The injured were taken to TriStar Hendersonville medical centre where they underwent a decontamination procedure.
Dr Berchaun Nicholls, an emergency room physician, said: “When they came in we set up decontamination tents and had them wash off and brought them into a room where they could be treated for their burns and given breathing treatments if they had any kinds of shortness of breath.”
He said they had superficial burns on their arms caused when boric acid and ethyl alcohol were mixed during the chemistry class experiment.
All the children and the teacher have been discharged from hospital and the school was due to reopen on Thursday.