Schools are cramped not just because of bigger class sizes but because pupils themselves have got bigger, according to teachers who say they feel as if they’ve “entered the land of the giants” when they walk into class.

Children are “crammed” into chairs and desks that are too small, and have to put their feet in the aisles, delegates at the annual conference of the NASUWT teachers’ union were told on Sunday during a debate on class sizes.

Part of the problem is cramped school buildings, many of which date to the 1970s, the conference was told. Meanwhile, three-quarters of teachers who were polled said class sizes were increasing, while more than a third (37%) said numbers had grown significantly.

“It’s not just the number of pupils in the room, it’s also about the increase in size of the pupils themselves,” said Elaine Paling, a national executive member representing Buckinghamshire, Milton Keynes and Oxfordshire. “When I look around [my school] now it’s not the fact that these rooms have shrunk it’s that the students have grown.”

In the 1970s, she said the average teenage boy was shorter, slighter and had smaller feet than today. “Now they are crammed into chairs and desks which are absolutely not suitable for them … and where do they put their feet? In the aisle.”

Another delegate, Fergal McGuckin, of North Down in Northern Ireland, agreed: “The size of pupils is increasing. I feel like I’ve just entered the land of the giants when I walk into some of my A-level classes. As someone who’s a standard five foot eight, I really do feel vertically challenged in those environments.”

Children in the UK have been getting taller and heavier for generations thanks to better nutrition. A survey of 5,000 seven to 16-year-olds in England, Wales and Northern Ireland, released by the Office for National Statistics and the Centre for Statistical Education at Nottingham Trent University, found that an average 15-year-old boy at the start of the 21st century, at 5ft 8in, was around nine inches taller than someone of his age would have been in 1837.

The average British foot is reported to have increased by two shoe sizes in the past 40 years, according to the College of Podiatry. In 1970, men’s feet were size eight on average and women size four, but now the average male wears a size 10 and the average female wears a size six.