The injured pupils of Christ Church High School, in Ashford, Kent, were all later sent home.A Kent Air Ambulance Trust doctor and ambulance staff assessed or treated a further 35 pupils at the scene while fire crews wearing breathing apparatus ventilated the building.
Pupil Georgina Fletcher, 11, said: "I was in a humanities lesson when we all started to smell something funny. We didn't know what it was and a lot of us started coughing.
"Then a teacher came in and we were all told to go down to the school medical room. Lots of people were panicking, some of my friends were coughing and some fainted.
"I was told to go home but two or three people from my class were sent to hospital." The pepper spray was released in the rear stairwell of a classroom block.A Kent County Council spokeswoman said lessons resumed after the incident and the school is now open as normal.Meanwhile, two parents were arrested at the school on suspicion of assaulting police. The men aged 32 and 37 were subdued with parva spray - a different substance from the illegal one used inside the school.