Officers were called to reports of an assault near Queen Elizabeth Park just after 12.30pm today (March 13).
London Ambulance Service crews also attended and found a boy who had been stabbed in the leg.
He was taken to hospital where his condition was assessed as non-life changing.
Three boys were arrested on suspicion of grievous bodily harm and remain in custody.
All pupils have been cleared from the building and no other injuries have been reported.
Officers are continuing to work closely with the school and a crime scene is in place.
We understand the incident happened at the Bobby Moore Academy in Sidings Street.
However, we have contacted the school to confirm.
A spokesperson for Metropolitan Police said: “Police were called to a secondary school, close to Queen Elizabeth Park at 12:33hrs on Thursday, 13 March following reports of an assault.
“Officers attended and three boys were arrested on suspicion of GBH. They remain in custody.
“The victim, a 15-year-old boy has been taken to hospital with a stab wound to his leg.
“His injuries are not deemed to be life-changing.”
