More than 21,000 school days were missed by pupils in St Helens last year so they could go on holiday with their parents.
The number is the same as 80 academic years being taken off by pupils in the space of 12 months.
Avis Gilmore, regional secretary of teaching union the NUT, said the prices of family holidays over the summer was to blame - particularly during the recession.
Speaking to the St Helens Reporter, which obtained the figures, Ms Gilmore said: "One of the issues we've taken up with the Government is the effect of the prohibitive prices in booking breaks away during the school holidays.
"But, ultimately, the onus is on headteachers to determine whether a family has valid grounds for pulling a child out of school."
St Helens Council chiefs say schools are allowed to give pupils up to 10 days' absence per year for family holidays.
Cases are reviewed by schools on an individual basis, and among those given special dispensation are parents who are unable to take holiday during the school breaks.
However, leave is not supposed to be given to allow parents to capitalise on cheap holiday promotions or if weather is poor during the school holidays. RV.