The Year 5 experiment was being conducted in the playground of the Sydney school in windy conditions on Monday, when a strong gust of wind caused the chemicals to explode onto 11 children and a teacher.

Two of the students received serious burns, with the others receiving superficial burns. All 12 people were taken to hospital.

A female student who received burns to her chest and face was airlifted to The Children's Hospital Westmead in a stable condition.

On Tuesday, a hospital spokeswoman said of the four children taken to Sydney Children's Hospitals Network - two at the Children's Hospital at Westmead and two at the Sydney Children's Hospital, Randwick, - three were discharged overnight.

She was unable to confirm the sex and age of the child who remains in hospital in a stable condition.

A student who was taken to Royal North Shore Hospital with burns remains there in a stable condition on Tuesday morning.

IN OTHER NEWS

Pauline Hardy, a mother of one of the girls injured in the explosion, took to Facebook to share her shock.

"Having had a daughter who suffered burns I felt very emotional hearing this on the news," she posted.

"Can only hope the worst affected children are not too deeply affected, and are given the wonderful care I know they will receive. Please, also remember the parents and the trauma they are now experiencing, also the poor teacher."

A male student of the school told 7New Sydney that the emergency incident was scary.

"I saw a lot of people crying and running around, it was kind of scary," he said.

NSW Education Minister Sarah Mitchell said the science experiment had gone "horribly wrong".

The students were conducting an experiment by mixing sodium bicarbonate and methylated spirits.

"This is a fairly routine experiment," Ms Mitchell told Seven's Sunrise program on Tuesday.

NSW Police, SafeWork NSW and the Education department will investigate the explosion.