Pupils and staff needed treatment by paramedics after a substance was sprayed in a classroom at a Preston school this afternoon.
Emergency services were scrambled to Fulwood Academy after reports that several people had been affected by the substance.
Police described the spray as an "irritant." It is believed both adults and children were suffering from eye and chest conditions,.
Two children were later taken to the nearby Royal Preston Hospital as a precaution, but were later allowed home after being checked by medical staff.
Police say they arrested two girls, both believed to be pupils.
An eye witness outside the school said a total of more than 20 vehicles from the emergency services were at the school, including four fire engines, eight police cars and three ambulances.
A spokesperson for Lancashire Police said: "We were called at around 2:45pm by the Ambulance Service to the report that a number of pupils and a member of staff had been sprayed with a substance at Fulwood Academy on Black Bull Lane.
"It is believed that a female pupil has sprayed the substance – believed to be some form of irritant - in a classroom.
"Two girls have been detained. Paramedics are currently at the scene treating those affected.
"There is not believed to be anyone seriously injured."
A spokesperson for the North West Ambulance Service added: "We were called to the school at 2.37pm to attend an incident."
In a message sent out to concerned parents, Academy Principal Philip Grant said: "You will have heard by now that we have had an incident in a class this afternoon which involved an
"The emergency services were immediately called as a precaution and is routine procedure.
"All the children directly involved in the incident have been checked over by the paramedics and later discharged into parental care.
"Thank you to those parents who have offered their help and support. The incident has now been closed and the school is open as usual tomorrow (Thursday)."