Every category of crime in schools in the Long Beach area of California has fallen dramatically this year - a startling achievement in a district known for Asian, Hispanic and African-American gangs and where a student’s home language may be any one of 46.

And the reason, say school staff, is simple. As of last autumn, all 56 elementary and 14 middle schools in the Long Beach Unified School District, with 58,500 pupils, required uniforms. And the number of school crimes recorded in the year fell from 3,242 to 2,074, down 36 per cent.

In a society proud of individual liberties, Long Beach took an extraordinary step last year when it compelled pupils to wear uniforms - the only public-school district in the country to do so. The results beg the question whether United States’ school reformers have overlooked a major force for change.

According to school district spokesman Dick Van Der Laan: “Incidents of assault and battery were down from 319 to 212, minus 34 per cent. Assault with deadly weapon: six to three. Fighting, 1,135 fights, two years ago, now 554. Down 50 per cent. Sex offences, 57 to 15. Robbery, 29 to 10. Extortion, five to two.”

Long Beach’s data is helping the growing popularity of uniforms in US school systems. Oakland is following suit, under the new California laws that Long Beach used. From Dade County, Florida, to Denver, Colorado, other school districts are either adopting or experimenting with uniforms.

“It has slowed it down immensely. It’s really effective, in a positive way. I tell other school officials that when I see them. I really believe it has a positive effect on individual behaviour,” said the district’s security officer, Sergeant-Major John Green. “I love it. It makes life a lot easier.”

The numbers of students suspended for serious conduct, like a fight or hitting a teacher, have also slumped. And some schools in Long Beach which have had compulsory uniforms for several years are citing anecdotal evidence that both attendance and even grades have improved.

Some caution, however, that uniforms are not a “magic bullet”. The American Federation of Teachers’ campaign to improve classroom discipline called “Lessons for Life: Responsibility, Respect, Results”, does not mention them. The risk of court cases has led some schools to introduce optional uniforms, and these have not been very successful.

Long Beach faces a law suit brought by parents and civil liberties groups to force it to provide free uniforms. That, says Mr Van Der Laan, would cost Pounds 27 million a year. The school already distributes Pounds 63,000 of free uniforms, clothing and even clean underwear and washing kits to pupils whose families are obviously in need.

His district has the largest population of Cambodians outside of Phnom Penh. It boasts socio-economic as well as racial diversity. Local TV stations have reported a new gang - the Asian Boyz - operating from the area. Uniforms effectively keep gang colours out of the classrooms.

Before and after lessons, school officials said, they make pupils less likely to commit crime because they know they can be spotted. And children are less likely to be the targets for gang members, either for wearing the wrong colours by mistake or as potential recruits.

Even without gangs, children tend to dress to type - whether skinhead, baggy pants, grunge or Beverly Hills 90210 - and uniforms can cut across these barriers. They may even change the attitudes of teachers who typecast pupils by what they wear.

Ronald Stephens, director of the National School Safety Centre at Pepperdine University in Malibu, backs uniforms as a crime prevention strategy. “Kids tend to behave the way they dress,” he said.