The charity BackCare blames school seats that are too low and poorly designed desks for a rise in children and teenagers suffering back pain.
The "ubiquitous plastic bucket chair" common to many schools in England should now be replaced with adjustable, tilting seats, it said.
Even old-fashioned wooden school desks with lifting tops were much better than today's classroom furniture because they were built at an angle.
Dorrie Willis, the charity's campaigns director, said she would not advocate children swinging on their chairs and sitting in "precarious"
But she said: "Tipping seems to put the chair into the the most healthy position for the back. This is often why kids might fidget.
"Unfortunately the plastic bucket seats do absolutely nothing for posture.
"Maybe 50 years ago it wasn't a problem. But lessons are more static now."
Children often have to sit still for double lessons lasting more than an hour without a break, as well as sitting in front of computers and taking less exercise than in the past - all of which contributes to back problems.
"It is an increasing concern," Ms Willis said.
Now BackCare is running a campaign for better classroom furniture, which can be adjusted for comfort.
The charity published a booklet which explaines that children who swing on their seats often did so to put themselves in a better position.
But generations of pupils have been told off for tipping their seats during lessons.
"As children, we all discovered for ourselves the trick of tipping forward on the legs of the chair," the booklet said. But because doing so often brings a reprimand, we lose sight of why we did it in the first place.
"By tipping the seat 20 degrees forward, we can avoid both bending and heavy loading the lower back.
"Besides, it feels so much more comfortable, because it's closer to an ideal balanced position."
Stage Systems, a British furniture firm, makes adjustable table and chair units at a cost of pound;110.
Anthony Hill, the founder of the company, said: "All the teachers that use this equipment say the handwriting of pupils has improved.
"People are beginning to understand that this is another Jamie Oliver effect - we need healthy food but we also need good posture."