They drive some of the world’s toughest teenagers to and from school and, like their colleagues in England (TES, April 23), many of them believe they get too little protection. Some school districts employ bus monitors to “ride shotgun”, but drivers often have to cope with up to 80 passengers on their own.

Some have been attacked with Mace, the anti-mugging spray, while others have been bombarded with nails and bolts whiledriving.

“School bus drivers are likely to confront behaviour problems that surpass anything encountered by even the most seasoned teachers,” Kentucky researchers told the AERA conference.

One driver interviewed by researchers from the East Kentucky violence prevention project said a student had pulled a knife on him. Another reported that a pre-school boy boarded his bus with a 10-inch blade. But it was middle-school children who generally behaved worst.

Some drivers reported bad behaviour to schools but others did not bother because they did not believe the culprits would be punished. The drivers said they needed more bus monitors, cameras and mobile phones.

However, the researchers suggest that training programmes for drivers and students may prove more helpful. “Students must be taught that good bus behaviour is as important as good classroom behaviour,” they said.

But the researchers acknowledge that the long journeys many children take in America’s 444,000 school buses make it harder to avoid confrontation.

One mother said her daughter had spent four hours a day on the bus during her four years at high school.

“My daughter figured out that travelling to and from school, she went around the world more than twice.”