A school has been evacuated and two people have been rushed to hospital after the suspected 'discharge of an aerosol'.
Paramedics treated more than 80 people following the incident at St Paul's Catholic High School in Manchester.
The pair taken to hospital were suffering from minor breathing difficulties, the North West Ambulance Service (NWAS) said.
Despite the huge emergency response, the school said parents do not need to collect their children early, the Manchester Evening News reports.
A message sent to parents by the school said: "Dear Parents/Carers. You may be aware that St Paul's has been evacuated this afternoon.
"This is in response to a suspected use of an aerosol in one area of the school.
"Emergency services have been called in order to support school with this matter.
"All pupils have been accounted for and as usual their response to this situation has been very mature.
"There is currently no need to collect your children early from school."
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An NWAS spokesman said: "We are dealing with an incident at a high school in Wythenshawe.
"We are at scene with a number of ambulances and clinicians and have assessed and discharged more than 80 people – we continue to do this.
"Two patients have been taken to hospital with minor breathing difficulties."
Before the ambulance service issued its statement, a Greater Manchester Police spokesman said: "We're assisting colleagues from @NWAmbulance and @manchesterfire following a report of a gas leak at a school today on Firbank Road in Manchester.
"Road closures are in place and the school has been evacuated as a precaution.
"No one is reported injured at this time."
