The chief inspector of Wales said this week that she understood the pressure on parents to take their children on holiday in term-time.
Susan Lewis, head of Estyn, the Welsh education and training inspectorate, said the long holidays in the academic year had little relevance in modern life.
"Schools are only open 200 days each year - no other organisation works like that," she told The TES.
Ms Lewis spoke out as her annual report, published this week, highlighted the continuing problem of truancy in Wales.
Attendance was found to be unsatisfactory in one-third of schools inspected this year, and the Welsh Assembly target of 92 per cent attendance was met by only one in six schools, compared with one in four last year. One in 10 schools did not meet the legal requirements for recording and reporting pupils' absences.
In her report, Ms Lewis said problems of truancy were exacerbated by the organisation of the school year.
"More and more parents are taking family holidays in term-time," she said.
"This is often because the school year does not fit in easily with the pattern of many parents' work commitments (and) the lower costs of holidays outside term-time."
She said it could take children several weeks to get back into gear after the long summer holidays.
"We have tremendous learning losses between one key stage and the next," she added.
Ms Lewis wants a radical review of the school year but she is unwilling to back any specific alternative structures such as the six-term year, which has been adopted in principle by 35 local authorities in England from September 2005.
"The number of terms is immaterial," she said. "But we need to ensure that schools are as accessible as possible to as many pupils as possible, so that learning can go on."
In England, the Office for Standards in Education refused to endorse Ms Lewis's call for a review of the school year, and said that was not within its remit.
Ofsted found that 21 per cent of secondary pupils in England had some unauthorised absence last year. Total attendance rates were 91.7 per cent at secondary level, and 94.2 per cent at primary.
A spokeswoman said: "Attendance in school is vital to children's learning.
Schools and local authorities should be working to reduce rates of absences."
Gethin Lewis, Welsh secretary of the National Union of Teachers, likened Ms Lewis's proposals to allowing hospital patients to turn up at any time just because a handful miss their appointments.
He said: "Holiday times will always be expensive for travel, even if there is a six-term year. Schools shouldn't have to change to suit parents' work-life balance."