Footage of the assault was then exchanged via mobile phones between some pupils at Kettlethorpe High School.
And the schoolchildren involved in making the film clip could face criminal charges.
Victim Mark Yates, 15, was left bleeding from cut wounds to his eye and ear and suffered blackouts after the attack which took place at the Standbridge Lane school at about 11.10am on Monday, January 29.
He then spent four-and-a-half hours at Pinderfields Hospital for treatment to his injuries.
His mum, Jill, 46, claimed the incident was pre-arranged.
She said: "It has been an organised public beating. There were about 40 pupils watching it happen."
Miss Yates also claims the film clip had been sent to her son's phone by other pupils trying to taunt Mark.
She said: "It makes me sick to watch it - it is horrendous."
At the time of the incident two police officers were already inside school on other business. They took Mark and another pupil to separate hospitals for treatment to their injuries.
A 15-year-old pupil was later given a formal reprimand by police for his part in the assault.
But Insp Karen Bailes has warned there could be further prosecutions at the school.
Insp Bailes said: "Those people who did the filming may have committed criminal offences.
"And since the incident we have learned that a film clip, captured on a mobile phone, has been sent around school to other pupils via mobile phones. We will be trying to identify any of those pupils involved."
Bill Cunningham, deputy headteacher at Kettlethorpe High School, confirmed they have a strict policy on the use of mobile phones in school.
If pupils are caught using them the phones are confiscated and then a pupil's parent has to come into school to collect it.
Mr Cunningham said: "We were obviously aware of the incident, and involved the police at the earliest opportunity.
"I am also aware of the video of the incident.
"The police are currently investigating the whole series of events, and we are co-operating fully with that investigation."