All schools in England will have to open for at least 32.5 hours a week, under government plans due to be published this week. The length of the school day is currently decided by the headteacher with the governing body in England.
Some 70% of schools are already open between 32 and 35 hours a week, with a further 9% open for longer. The rule - which starts next September - will hit the 14% of schools thought to be open fewer than 32 hours a week.
The data comes from a July 2021 government survey which asked schools how many hours a week pupils in years 3 or 9 had to be in school. About 6% of schools did not teach pupils in this age bracket.
The government said the change would ensure children had a fair chance to engage with a range of subjects - as well as any catch-up support - wherever they lived. The 32.5 hour school week is equivalent to 08:45 to 15.15, with the government arguing that a child with a school day shorter by 20 minutes a day would lose two weeks of schooling over the course of a year.
But education unions said the new requirement would make little difference as most schools already met the threshold. They suggested schools that did not could be in rural areas which face greater transport challenges.
Education Secretary Nadhim Zahawi said the change was part of a wider ambition in "creating opportunity for all, with strong schools and great teachers for every child". He is due to set out the government's wider plans for schools in England this week in a White Paper, alongside a review of the support available for pupils with special educational needs and disabilities (SEND).
The plans include a target for 90% of pupils leaving primary school to be at the expected level for maths and English by 2030. In the latest available figures from just before the pandemic, 65% of pupils in year 6 reached that level.
Labour shadow education secretary Bridget Philipson said after two years of "pandemic chaos" and six years since the government's last schools strategy, the plans would leave parents, teachers and pupils questioning where the ambition for children's future is. The National Education Union's Kevin Courtney said schools and pupils had been left battered and bruised by the pandemic and a more sophisticated approach was needed.
Paul Whiteman, of the headteachers union, the NAHT, said: "Simply adding five or 10 minutes to a day is unlikely to bring much, if any, benefit."