Eleven children and a teacher have been injured in an explosion caused by a science experiment gone wrong at a Sydney school.

The victims suffered burns during the accident at Manly West Public School in Balgowlah on the city's northern beaches just after 1pm.

Two of the students in the year five class – who would be aged 10 or 11 – suffered serious burns to their face and chest after the class mixed what's believed to be baking soda and methylated spirits in a test known as "the worm".

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The experiment took place outside on the sports field, with the strong winds blamed for the mishap.

An 11-year old girl was airlifted to the Children's Hospital at Westmead in the CareFlight helicopter with serious burns.

The other was transported to the same hospital but in an ambulance with a specialist medical care team on board.

Both are now in stable conditions.

"It was meant to be a little science experiment but it got a bit out of hand," one student told 9News.

One mother said it was "very distressing" to see a helicopter land on the school field.

The remaining nine students were all taken to hospitals including Royal North Shore but had "superficial burns", according to NSW Ambulance.

The male teacher was rushed to Northern Beaches Hospital with burns.

"All of them, apart from the two in a serious condition, were walking and were stable with relatively superficial injuries," NSW Ambulance Acting Superintendent Phil Templeman said.

"Today's high winds have impacted the experiment and blown some of the materials around.

"The children have sustained burns to the upper body, chest, face and legs. Thankfully, the majority of the students were not badly injured."

A CareFlight helicopter, over a dozen ambulance paramedics and five police cars all responded to the accident.

Investigations, including by the Department of Education, are under way into what happened.