Ambulance NSW says paramedics and the CareFlight helicopter were deployed to Manly West Public School at Balgowlah, on Monday afternoon, responding to 12 patients with burns.
Eleven students, believed to be around 10, were treated at the scene before being taken to various hospitals.
All of the children are in a stable condition, however two suffered serious burns and were transported to hospital under the care of specialist medical teams.
Nine other children suffered superficial burns and were also taken to hospital for treatment.
The two children with serious burns were taken to Westmead Children's Hospital, five children are being treated at RNS Hospital and four are at Northern Beaches Hospital.
An adult who also received superficial burns, did not require hospitalisation.
There are reports the students were conducting a science experiment, mixing sodium bicarbonate and methylated spirits.
Police officers initially responded to what was labelled a HAZMAT incident shortly after 1pm on Monday.
Concerned parents reportedly gathered outside the school gates for news of what occurred, surrounded by ambulances and police vehicles.
10°C