The obesity crisis is forcing school authorities to look for larger and stronger furniture in Wales, UK
Figures show that 22 per cent of boys and 17 per cent of girls in Wales are overweight or obese, with almost one in five 13-year-olds falling into the same category - higher than in England, Scotland and Ireland.
A report by furniture suppliers, which measured 1,400 pupils, found that children are more than an inch taller and several pounds heavier than a generation ago.
Philip Dixon of ATL Cymru, the union for teachers and university lecturers, said: "We've got to make sure that children are comfortable in the classroom.
"But the underlying thing is we have to tackle childhood obesity because it leads to all sorts of problems. We've got to tackle obesity. We can't go on like this.
"Teachers are reporting a definite problem. Our members are concerned in the rise we have seen in childhood obesity and that's certainly having an impact in the classroom," he added. "We have to act over this. Statistics show that children are overweight and that is why we have to make allowances.
"We need to make arrangements to deal with the effects of this and furniture needs to be adapted, because it has health and safety implications as well."
A Pembrokeshire County Council spokesman said: "We have carried out some consultation with pupils and a small number find their chairs too small. In the light of the survey, we will ask for furniture samples from the supplier for the next new school we build to ensure that desks and chairs suit all pupils."
Researchers found that children's arms and legs are longer and rounder and their legs are also longer. As a result, suppliers are making chair legs sturdier and chair backs stronger to cope with bigger children.
The Furniture Industry Research Association (FIRA) study has led to new EU guidelines and all new fittings are set to add up to an inch to tables and chairs, says Laura Wright, writing in Wales On Sunday.
Graham Hirst, head of sales and marketing for Baglan-based Remploy Furniture said: "We've been making educational furniture since 1946 and have seen pupils' sizes change with issues such as childhood obesity and they're growing up fast and they're a little bit wider as well. There is a need to make a change.
"In primary schools, it might be that the size of furniture that suited a seven year old 10 years ago, would now suit a five or six year old. We are heavily involved in the migration from old size furniture to the new sizes.
"But the message hasn't really reached schools yet.
"It's reached manufacturers and councils but when we talk to schools, they haven't heard of the new standards. Inevitably, they will do."