The National Association of Head Teachers wants the so-called "three for two" rule that allows councils to put three children under 14 in seats meant for two to save money to be scrapped. David Hart, general secretary, said that pupils were treated as second-class citizens. "The rules and regulations governing the transport of cattle seem to be more rigorous than those governing the transport of children," he said.
The union said that children were bigger now than 50 years ago and the rule caused dangerous overcrowding.
In 2001, 642 children were injured going to and from school, one of them fatally, said the NAHT. The numbers had decreased since 1999, when there were 874 casualties, including 39 serious injuries.
But the NAHT said the Government should close the loophole that allowed homeschool buses to be less safe than vehicles used for trips. Since 1997, buses used for school trips have by law had to have a seat with seatbelt for every child.
But in buses that ferry children between home and school it was often the case that some pupils ended up standing, and were then more likely to misbehave.
Often the only adult on board was the driver. Although he, or she, was responsible for the children's safety, they were there to drive and could not be expected to supervise youngsters at the same time, the union said.
Mr Hart is writing to the ministers responsible in England, Wales and Northern Ireland demanding change.
"Government must take its share of the blame. Regulations brought in 50 years ago are patently out of date and in dire need of urgent replacement.
Government encourages greater use of buses but does nothing to tighten up the law surrounding their use."