A 13-year-old boy was tied to a tree with masking tape before being set on fire in a "happy slap" attack.
Kyle Parker was tied to a tree which was then set alight
Kyle Parker told police he was assaulted after going to a wooded area in the grounds of Smithills School in Bolton on Friday lunchtime.
He said a group of about eight boys attacked him and filmed it on a mobile phone, before setting him free and letting him go back into school.
Detectives have seized several mobile phones as part of their inquiry.
A Greater Manchester Police statement said: "The 13-year-old boy has been video interviewed and officers are continuing their inquiries.
"As part of their investigations officers have seized a number of mobile phones as well as other items allegedly used in the incident."
Karl's mother said she was 'disgusted'
Kyle told the BBC his attackers grabbed him and tied him to a tree.
"They took my school tie off and tied it round my feet," he said.
His mother, Maxine Lever, said she was "disgusted".
"He could have flared up into flames," she said.
"Someone is going to end up getting killed."
A statement from the school said the matter had been referred to the police.