Pupils aged just seven too obese to fit into their uniforms
Children as young as seven are having to get 'adult' size school uniforms.
And as a result, parents are left with higher bills because the clothes don't qualify for zero-rate VAT.
Department of Finance rules allowing for a VAT exemption on children's clothing and shoes also extend to school uniforms.
The zero rate applies to clothing up to a size that the Revenue Commissioner deems would be no bigger than what a 10-year-old of average build would wear. This is set at sizes up to and including a 32-inch chest or a 26-inch waist.
But school uniform manufacturers and suppliers say the guidelines are out of date with the height and weight of modern children.
The Irish School Wear Association (ISWA) spokesperson Karen Grant said they were "aware of a plethora of cases where children as young as seven require uniforms of a size too big to qualify for VAT".
The ISWA has made its case in a pre-Budget submission to Finance Minster Michael Noonan, citing figures from a 2012 Safefood report.
The report quotes data from national surveys conducted in 1948 and 2002, which show that children's heights increased by 23cm, about nine inches in boys, and by 15.6cm, about six inches, in girls, in the intervening years.
The increase in weight was proportionately higher, with 14-year-old boys weighing 24kgs more in 2002 than they did in 1948.
The survey comparisons also highlighted an accelerating problem with obesity affecting children.
Ms Grant said it was "clear that the measurement restrictions in place for the zero percent rate did not adequately cover today's schoolchildren".
VAT was introduced in 1972 and Ms Grant said the regime disadvantaged parents of bigger children, mainly because the tax law had not kept up to date with the increase in children's sizes.
"The zero rate should be expanded to ensure that all school uniform items, for school-going children of any age and size, were not subject to VAT. The main issue for many parents in these days of austerity is the cost of living and adding 23pc tax to the cost of a school uniform, due to 'lazy legislation', must be stopped," she said.
The tax rules on zero rating for children's clothing in the UK are more lenient and cover sizes to fit up to a 33-inch chest and a 27.5-inch waist for boys, and a 33.5-inch chest and 27-inch waist for girls.
Weight-loss specialist Dr Eva Orsmond said many parents of overweight children are "living in denial".
"They are worried about making an issue of their child's weight, because they think their son or daughter will suddenly become anorexic, or develop some sort of eating disorder. In reality, that's very unlikely.
"Many parents convince themselves extra weight in a child is puppy fat."