Explosion at Sydney school investigated

An investigation is under way after an explosion at a northern Sydney primary school left 11 children and a teacher with burns.

NSW Education Minister Sarah Mitchell said two children are still being treated after a science experiment went "horribly wrong" at Manly West Public School on Monday.

Two of the year 5 children have serious burns, while nine others and a teacher have have superficial injuries.

The students were conducting an outdoor science experiment, mixing sodium bicarbonate and methylated spirits.

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

"We have really strict measures in place around work, health and safety and how we manage these experiments in school," she said.

"Obviously, we will investigate what went wrong."

The school staff had done a great job supporting students in "very upsetting circumstances", she said.

"We are thinking of students who are injured and wishing them a speedy recovery and we will make sure we have supports in place at the school, including extra counselling."

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