Children as young as seven are having to wear 'adult' size school uniforms because they are overweight.
As a result, parents are left with higher bills because the clothes no longer 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 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 step with the height and weight of modern children.
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".