Three pupils have been taken to hospital after an incident at a school where a "noxious substance" was used.
A disturbance, involving a group of youths, took place at Sydney Russell School in Dagenham on Wednesday afternoon, police said.
Three boys aged 12, 13 and 15 have been arrested on suspicion of causing grievous bodily harm.
Two of the injured pupils have been discharged while the third remains in hospital.
The three victims are believed to be two girls and a boy.
All those involved were pupils at the school, the Metropolitan Police said.
The school was rated outstanding by Ofsted at its last inspection. The school itself has yet to comment on the incident.