Delegates at the National Association of Headteachers conference passed a motion calling for a five-term year and 37-hour working week, abandoning the statutory 1,265 hours of teaching.
David Gray, head of Babbacombe CE primary in Torquay and Michael Rumsey, head of Sampford Peverell CE primary in Tiverton, Devon, proposed a working day that began at 8.15am and ended at 4.45pm, but with teaching finishing at 2.15pm. They said it was time society stopped regarding schools as child-minding centres.
After 2.15pm, they said schools could become community learning support centres and pupils could participate in after-school activities, but teachers would not be obliged to run them.
This change to the school day would create time for marking, training and other requirements, leaving evenings and weekends free for teachers to spend time with their families.
An NAHT survey has shown that nine out of 10 heads would consider changing the present three-term structure.
The new system would mean five eight-week terms with two weeks between them, and a four-week break in the summer.
The challenge to the traditional three-term year will be spearheaded by the Greensward School in Essex, which plans to pilot the scheme from January 2000. It will be one of the first schools in the country to carry out the reform.
East Sussex is also consulting about introducing a five-term year (see box).
Chris McDonnell, headteacher of Fulfen primary school, Staffordshire, an ardent supporter of the change, said: "The three-term year was designed in the 19th century to meet the needs of an agricultural society. It's time we woke up to the modern world and instituted a system that reflects the needs of the 21st century."
He argued that the long summer break has a detrimental effect on children, who "forget what they have learned during the year". The new system ensures that they are never out of school for more than four weeks.
* Chris McDonnell will lead the arguments for a five-term year at a seminar on June 23 at the Institute of Education, London.