Buses can often be overcrowded, a union has warned

Since 1997, it has been illegal to run an organised school coach trip without pupils being strapped in.

But a lack of national guidelines for daily bus journeys is an "abdication of responsibility", the National Association of Head Teachers said.

It also warned against crowding "up to 100" pupils on board.

'Loopholes and confusion'

An NAHT letter to Education Secretary Charles Clarke reads: "The government is aware of the problem but lets children down by doing too little.

"There are loopholes in the law and confusion about where responsibility lies."

Some councils operate a "three-for-two" rule, where three children under the age of 14 use a seat designed for two adults. This should also be made illegal, the NAHT said.

The letter goes on: "The regulation...was written some 50 years ago. Children today are a lot bigger due to better nutrition and diet.

"They also take more equipment to school in bigger bags. In practice, pupils often have to stand during the journey, sit on laps or fight for space on crowded seats."

When pupils stand and where belts are not used, youngsters are more likely to misbehave, as they are free to move up and down the bus, the NAHT added.

'What about the drivers?'

The letter says: "Most home-to-school buses are unsupervised.

"Where else would we crowd almost 100 youngsters into a confined space and leave them without adult control for up to an hour?

"In law, drivers are supposed to manage passenger behaviour but how can they when they are supposed to be driving the bus?

"Drivers themselves are equally concerned about the challenging circumstances they face."

The NAHT warned that transport contractors often used the worst buses in their fleets for the school journeys, arguing that children would damage more up-to-date models.

Hard-up local authorities were also likely to choose cheaper options.

Liberal Democrat education spokesman Phil Willis said: "For too long school transport has been regarded by many local authorities as a 'fag-end' service - cut to the bone with minimum safety requirements.