Next week researchers from the Student/Teacher Achievement Ratio (Star) Project at Tennessee State University will explain their findings to a British audience at a conference organised by the Institute of Education in London. Many parents and teachers will be hoping the team can be as persuasive here as with their own legislators.
Not long ago the state of Tennessee had some of the largest class sizes in the United States. Campaigners led by the teachers' union the Tennessee Education Association had fought with little success to get numbers down.
Cavit Cheshier, who has just retired as the union's leader, says: "It was a long, hard battle and the obstacle was that there was no proof that class size had an effect on standards."
So Mr Cheshier and another campaigner, Helen Pate-Bain, a Nashville teacher who is a former president of the National Education Association, the national teachers' union, determined to get that proof. It was their pressure that persuaded Tennessee's political leaders to pay for Star, the most ambitious study of the link between class size and standards ever undertaken.
Star monitored the progress of 7,000 children for four years as they moved from kindergarten through to third grade (ages five to eight). To strip out all other variables, they allocated pupils randomly to either small classes (13 to 17) or regular classes (22 to 25). A third category comprised regular-size classes with a teachers' aide. Every one of the 80 schools involved had to have one of each type of class, to reduce variations due to different school types.
By far the biggest cost in this expensive project was the salaries of the extra teachers required to create smaller classes. The state of Tennessee stumped up the $12m needed. That sort of bill puts extensive, longitudinal research of this kind beyond the means of everybody in Britain, except the Government. Ministers, unfortunately, are too wary of the possible results to fund such research.
What did Star find? A variety of tests, such as the Stanford Achievement Test and the Basic Skills Test, were used to monitor the three categories of class year by year. The small classes consistently outperformed the others. Dr Barbara Nye, director of the project, says the difference was "educationally and statistically significant". The difference between the two categories of regular-size classes was not significant.
The small classes scored better in all subjects, at all ages and in all locations (inner-city, urban, suburban and rural). The biggest advantage for the smaller classes came in first grade (for six-year-olds), and the benefits were maintained and enhanced at every subsequent grade level. Ethnic minority children gained particularly.
The initial Star research ended in 1989 but a follow-up project, called the Lasting Benefits Study, has monitored the subsequent progress of the children involved. The children returned to regular-size classes at the age of nine. The follow-up study shows that the group that began in small classes was still showing the benefits at the age of 12.
Ms Pate-Bain found the research proved what she had always suspected: that class size does make a difference, especially in the early years. She says: "I always thought it was there. Now I know I can prove it, even to the most doubting Thomas."
Having done the research, campaigners in Tennessee have achieved their aim. The state recently passed a Basic Education Plan, which gives schools the funds to restrict to 20 all classes for pupils aged five to eight. Mr Cheshier says the Lasting Benefits Study was particularly important as it persuaded the secondary school lobbyists that money spent in the elementary schools would have long-term benefits for high schools too.
Will the Star team persuade British legislators of the value of small classes? I asked the Schools Minister, Eric Forth, whether he accepted the Star findings. He pointed to the cultural and educational differences between the countries and said: "Simply looking at what other countries are doing and following it slavishly is not the answer." Ministers here prefer to stress other factors that also determine standards, such as teaching methods and classroom organisation.
However, American teaching methods are actually quite similar to those used here. Dr Nye observes: "Children learn in similar ways and I think this research is very applicable to other countries." Many teachers and parents are likely to agree.
The author is the BBC's education correspondent.