A move towards nine-hour school days and longer terms could leave many children exhausted and too tired to learn, it has been suggested.
Under plans reportedly being examined by the Conservatives, school hours would be extended, with pupils staying until around 6pm, while school holidays would be cut almost in half, to around seven weeks a year.
But teachers' leaders warned that many children are already tired at the end of the school day, and need regular breaks to help them process what they have learnt in the classroom.
State school pupils usually get two weeks off at Christmas and Easter as well as six weeks in the summer, and three week-long half-term breaks.
School days usually run from between 8.30am and 9am, sometimes earlier, to 3pm, or 3.30pm.
Proposals drawn up by David Cameron's former policy chief Paul Kirby would see school days extended to run from about 9am until 6pm, while holidays would also be reduced to seven weeks.
Mr Kirby told The Sun that it would solve a wide range of issues, "transforming the lives of most households in the UK within two years".
It was suggested the extended days could reduce youth crime, boost education standards and prepare children for the world of work by getting them used to full days. It could also allow parents to return to full-time work.
The Sun said Tory ministers were examining the plans, which would apply to all children in England between the ages of five and 18, in time for the party's 2015 general election manifesto.
Mr Kirby said: "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.
"It would also go a long way to solving the crisis around childcare affordability, a major issue for many parents."
But Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), said: "Most primary children and many older children are exhausted by the end of the current school day, and exhausted by the end of a school term.
"For children to learn effectively, they need regular breaks to give them time to process what they have learned. There is no evidence that extending the school day would raise exam results or help children achieve more at school - children will just end up too tired to work."
Christine Blower, general secretary of the National Union of Teachers (NUT), said the move would not be supported by the majority of parents.
"Children are not an inconvenience to fit in around work. Equally, education should not be viewed as a production line.
"For many children, spending such a long period in school will be counterproductive."
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), said: "This isn't really a new idea. Most schools already operate extended hours and open well before 9am."
Academies and free schools in England already have the freedom to set their own hours and term times.
From September next year it is proposed that this will be extended to schools still under local council control.
It would mean that councils would no longer be able to tell their schools when terms should start and end, paving the way for individual schools to introduce longer terms and cut school holidays.
Moves to change term times and lengthen school days have already proved controversial.
Last year, a poll of parents found many believe that allowing schools to set their own term times will make it tougher for families with children at more than one school to arrange childcare and family activities.
Some teaching unions have also argued that school staff and pupils already spend long hours in the classroom.
Education Secretary Michael Gove has previously called for longer school days and terms, warning that the current system is out of date and fit for the agricultural economy of the 19th century.
In a speech last year, he claimed that pupils are at a ''significant handicap'' compared to youngsters in East Asian nations who benefit from extra tuition and support from teachers.
And last month, Ofsted chief inspector Sir Michael Wilshaw indicated he would like to see changes to the school calendar.
Asked in an interview if teachers had too many holidays, he said: ''I think the six-week holiday is too long.''
A Department for Education spokesman said: "We are already giving all schools the freedom to set the length of the school day and term. Many academies and free schools offer extended opening hours, and we want more schools to take up these freedoms.
"We will obviously consider recommendations for further reforms."