More than 1,000 schools across the country have extended the calendar by adding at least 10 days to the year or by lengthening each school day by half an hour, research has found.
The average school year in the US, as in Britain, is 180 days. This is much lower than countries such as Japan where many schools have years of well over 200 days.
However since a report titled A Nation At Risk, commissioned by the then President Ronald Regan and published in 1983, US schools have been urged to act upon a recommendation that the school year is increased.
The National Centre on Time and Learning, a non-profit research group in Boston, says that it is only recently that progress has been seen.
The NCTL will release a report next month which will show that more than 1,000 schools across America have either gone to a 190 day calendar or have lengthened the school day from the standard six-and-a-half hours to seven.
The schools increasing the most are charter schools – similar to free schools in the UK in that they are not controlled by local authorities but are free to attend. One charter school in Atlanta, KIPP West Atlanta Young Scholars Academy, has a nine-and-a-half hour day and a 219 days a year calendar.
The increases come against a backdrop of poor standards of academic achievement in the US. The country is 23rd in maths and 31st in science when compared to the performance of students from 65 other top industrial countries.
The other reason for the increase, according to Chris Gabrieli, the chairman of the NCTL, is that standard testing in maths and English has left schools with little time to devote to other 'enriching' subjects such as art, sport, music and areas where children are not tested and schools judged by results.
Mr Gabrieli added: "The primary driver of dissatisfaction with the current school schedule is the realisation that so many students were doing quite poorly against academic benchmarks .
"The second issue is that a short school day combined with a big increase in the amount of time devoted to maths and English, the test subjects, means that the amount of time diminishes for the opportunity to have a well-rounded education."
Advocates of a longer school day include Arne Duncan, the US secretary of education, who says that the traditional school calendar is outdated.
"The fact that our calendar has been based on the agrarian economy when almost none of our kids work in the field anymore doesn't make any sense whatsoever," he said.
