The 12-year-old attacker, from Kettlethorpe High School, pulled out a kitchen knife and held the blade against his victim's throat after a row.

Bus driver Geoffrey Carr heard the shout "he has got a knife" and immediately stopped the school special bus as it turned into Chevet Lane.

He leapt from his cab and disarmed the boy after wading through the crowd of terrified youngsters who had gathered at the back of his bus.

Mr Carr, 67, played down his actions in preventing a tragedy.

He said: "I could not just sit there when someone was flashing a knife about.

"When I got to him he was just screaming with the knife in his hand."

After recovering the knife Mr Carr, of Harewood Avenue, Upton, called the police, but the boy ran off.

It was the first time Mr Carr, who has been driving coaches and buses for about 40 years, had taken the 506 service.

He said: "I have seen the odd fight but nothing like this."

The victim suffered scratches and was taken to hospital.

Tudor Griffiths, headteacher of Kettlethorpe High School, said: "We are aware the police are investigating an incident that took place away from the school. A 12-year-old pupil has been removed from school and it would be inappropriate to comment further at this time."

Nigel Featham, managing director for Arriva Yorkshire, said: "The safety and wellbeing of our customers and drivers is paramount and I am glad to say incidents like this are extremely rare. We will be working closely with the police on their investigation.

"While our policy is that employees should never put themselves in danger, we are proud of Geoffrey Carr and the professional manner in which he dealt with the situation."

The incident took place at about 3.25pm on Thursday, May 14.

Detectives want to speak to a 13-year-old boy who is about 5ft 6ins tall and had black medium-length hair.

Anyone with any information should contact Wakefield CID on 01924 295602 or Crimestoppers, in confidence, on 0800 555111 and callers could receive a cash reward.