But if that rule applies to school class-size policies, infant teachers in England and Wales will have some reason to celebrate.
While they are still waiting for the British Government to fulfil its promise of keeping classes down to 30 pupils, many US states are insisting that infants should be taught in classes of 20, or even 15.
As Professor CM Achilles, of Eastern Michigan University, told the AERA conference, no fewer than 27 US states have either passed legislation reducing class sizes, have debated the topic seriously or are testing the impact of smaller class sizes.
The state of Tennessee pioneered the new wave of class-size reductions in the 1980s. But an even more ambitious experiment is under way in California. The state government has since the autumn of 1996 been offering $650 a year per child to school districts that keep classes for five to eight-year-olds down to 20 pupils.
Although the grant does not meet all the costs only a handful of districts have rejected the offer and the state is now spending more than $1 billion a year to educate 1.9 million infants in small classes.
Florida is also proposing to continue its three-year-old policy of limiting infant class sizes to 20. Nevada has reduced classes to 15 in selected schools. Virginia has a long-term goal of reducing infant class sizes in schools catering for substantial numbers of at-risk children, and Wisconsin has cut classes to 15 for children aged five to seven in "low wealth" schools.
It is the California scheme that has, however, commanded most attention over the past 18 months. Although the policy has public support, it is feared that the recruitment of more than 4,000 unqualified teachers could undermine the scheme, particularly in poor urban areas.
Dr Marilyn Korostoff, of California State University, Long Beach, who has been tracking the impact of the reform, said that infant teachers were nevertheless generally delighted. "I'm ecstatic. I can't believe this is happening," was a typical reaction.
However, they were working hard to prepare children for district assessments. "We are really feeling the pressure to perform," one teacher told her. "If our scores aren't up, this could all go away."
But principals had been put under much greater pressure, having had to find extra staff and accommodation at very short notice. "It seems as if this policy has hidden landmines at every turn," one confided.
Some veteran teachers had also become resentful of the time they had to spend with the newly-recruited teachers as they wanted the freedom to enjoy their own 20:1 experience. Teachers who were having to cope with larger classes of older children had also begun to complain about the disparities in workload, although some were looking forward to a new intake of children with more developed skills.
Contact: Dr Marilyn Korostoff e-mail firstname.lastname@example.org