They would also aid the national economy by expanding the workforce and prepare pupils for the tough jobs market by getting them used to working full days.
Tory ministers are examining the proposals, which would cover all primary and secondary pupils from five to 18 — and would be strictly enforced by law.
The Sun has been told the idea is “a serious contender” for the Conservatives’ 2015 General Election manifesto.
Under the scheme — drawn up by David Cameron’s former No10 policy chief Paul Kirby — the average school day would run from around 9am until 6pm, 45 weeks of the year. Christmas, Easter and summer holidays would be reduced to little more than two weeks.
Mr Kirby said it would go a long way to solving a host of big issues for voters — “transforming the lives of most households in the UK within two years”.
He told The Sun last night: “This is a once in a generation reset that wouldn’t detract from the current school freedom agenda. It also involves dramatically expanding what schools actually do — into sport and other activities.
“All the lessons from abroad where this has happened previously, such as the United States, show that elongating the school day takes much of the stress away from teachers and pupils.
“It would also go a long way to solving the crisis around childcare affordability, a major issue for many parents.”
A massive stumbling block would be persuading left-wing teaching unions to sign up to so many more working hours.
They would demand billions more in pay for the UK’s 440,000 state school teachers. But backers of the plan argue it would save a fortune in the long term.
Ministers would pay out far less in child tax credits, plus childcare vouchers and land hundreds of millions more income tax from extra working mums.
Under Coalition reforms, schools can already set the length of their days and terms. A handful of pioneers have ten-hour days, but most refuse to keep kids after 3.30pm, claiming funding problems.
A source close to Education Secretary Michael Gove said: “We welcome all ideas on how to make schools stay open for longer, including this one.”
But a National Association of Head Teachers spokesman said: “Most schools provide extended services for children and the nature of those services are best determined by those children’s needs.”
TEACHING experts last night said the plan was a good chance to help kids.
The Campaign for Real Education — formed by parents and teachers — said: “Children do benefit from being in school.”
Chairman Chris McGovern added: “Some from deprived backgrounds do suffer over long summer holidays.”
Others remained unconvinced. Education union ATL said: “Children need breaks to refresh them. A lot of them are shattered at the end of the day.”
ROOFER James Doyle, 42, who lives with wife Karen, 37, and his kids Jacob, 13, and Olivia, 11, in Plymouth, Devon, said: “It would be quite a good idea as I have to finish work early to collect my kids.
“We have to pay up to £280 a week in the summer for my kids to go to a summer club so me and my wife can work. And my son Jacob thinks the holidays are miles too long. It will help prepare kids for working once they finish school.”
Nurse Joanne Hope, 44, who lives with kids Martin, 15, and Rachel, 13, in Plymouth, added: “Longer school days would prepare children for adult life. Schools should be more in keeping with parents’ work.”
ARCHIE Ferris, 15, who attends Backwell School in North Somerset, said: “It’s not a good idea as it would be hard to concentrate.
“We already have to sit around for eight hours. I play a lot of football after school and if we have to stay longer I’d really struggle to train as much.
“I know I’d mess around a lot more if I had to stay in school even longer — a lot of people would be the same, probably out of boredom. It’s not fun being in school as it is.”
Millie Pye, 17, an A-level student at Backwell School, added: “People just wouldn’t have the concentration span.
“I also think it’s silly to reduce holidays. We need them to relax and catch up with work.”
HEADTEACHER Kieran Earley, runs Devonport High School for Boys in Plymouth and said: “The idea that the school day starts at 9am and ends at 3.30pm is not a fair representation as schools already offer a great deal in terms of breakfast and after-school clubs.
“Teachers work incredibly hard already.”
Jackie Francis, 50, who teaches special needs kids at St Ivo School in St Ives, Cambs, said: “The day doesn’t finish when students leave. Often we stay till at least five o’clock and have to do marking when we get home.
“This would pile extra pressure on us. We’ve a right to family time too.”