A schoolboy suffered a fractured skull and a bleed on the brain after police were called to reports of a man attacking people with a metal pole in south London.
The teenager was struck as he left Dunraven School in Leigham Court Road, Streatham, at 3.30pm on Wednesday.
He is in hospital surrounded by family in a life-threatening condition, police said.
The attacker is alleged to have hit the boy in the head with the pole before moving on to several other pupils and a teacher.
The other children had minor injuries.
A 35-year-old man has been arrested on suspicion of grievous bodily harm.
He was treated in hospital for minor injuries after being tackled by a police liaison officer based at the school.
Lambeth Police wrote on Twitter: “A 13yo boy was found with a head injury. He was treated at the scene before being taken to hospital.
“The boy’s condition has been assessed as life-threatening.”