Eleven school students and two teachers have been overcome by toxic fumes during a science class in Sydney's south-west.
Worried parents raced down to Moorebank High School just before the school day finished after hearing sirens.
When they arrived, students and teachers were being treated by emergency crews with many being wheeled into ambulances.
It appears to have been a year seven science experiment gone wrong. A sulphur-based chemical overheated giving off fumes that quickly overwhelmed the class.
All up, 13 people were taken to hospital with breathing difficulties.
They are set to make a speedy recovery, but it was a frightening experience for those caught up in the drama.
"Somebody put chemical on the Bunsen burner and oil and chemical went everywhere," student Gabriella Fonseca said.
"People had to go to the hospital and we had to evacuate the whole building of the maths and science block."
Student ignited homemade device
There was also a second school emergency involving dangerous chemicals today.
A student ignited a homemade device in a stairwell at Plumpton High School this morning.
Fumes forced three classes to be evacuated but no one was hurt.
A teenage boy has been taken into custody.