The National Association of Head Teachers said rising divorce rates meant children often took two holidays, one with each parent. It also blamed work pressures, which meant parents took holidays at employers' convenience, and an increase in the number of families taking two holidays a year.
In addition, some ethnic minority families visited Pakistan, India or the Caribbean in November and December or between January and March.
David Hart, the association's general secretary, said: "The law permits pupils 10 days' authorised absence per school year, but parents are increasingly taking advantage by treating this as a right.
"It iswrong that parents cannot, or will not, find time for holidays within the existing 14-week `envelope'. The travel industry's promotion of cheap family holidays in term time is a major contributory factor.
"The law is out of date and totally at odds with the Government's drive to raise standards." He added that parents were increasingly ignoring schools' requests not to take their children on holiday during term, meaning children might miss national curriculum tests and fall behind with vital work. One secondary school had recorded more than 1,000 days lost to holidays last year.
The Department for Education and Employment is holding talks with the travel industry about the problem.
A spokeswoman for the Association of British Travel Agents said: "Tour operators are not actively promoting cheap family holidays in term time. They are simply in the brochures at different prices.
"It is a question of supply and demand; the six weeks of the summer holiday tend to bewhen most people want to go."
Margaret Morrissey, press officer for the National Confederation of Parent Teacher Associations, said the Government attitude was contradictory. It allowed parents to take children out of school for two weeks a year, but then penalised schools by including these absences in the league tables.Reuse content