Officers from Barking and Dagenham are investigating after three teenagers were injured at an east London school.
Police were called by the London Ambulance Service to Sydney Russell School in Parsloes Avenue, Dagenham, at 14:11hrs on Wednesday, 22 February to reports of a disturbance involving youths where a noxious substance had been used.
Three people, believed two females and one male, were taken to an east London hospital for treatment. They have all since been discharged.
Three males, aged 12, 13 and 15, were arrested on suspicion of causing grievous bodily harm. They were taken to an east London police station where they remain in custody.
All involved were pupils at the school.
Anyone with information is asked to contact police via 101.
To give information anonymously contact Crimestoppers on 0800 555 111 or visit crimestoppers-uk.org.
Enquiries into the circumstances continue.