Principal Steve Baker, pictured with firefighters, said: "It is a real testament to teachers and the children that they came to an open evening after being discharged from hospital.
"We carried out an investigation. We looked at cleaning materials and checked in the science lab but could find nothing to explain what happened.
"The smell was a bit like burning tyres."
The gas was released in a stairway just as pupils returned from morning break. Two people were arrested and released on bail following the incident.