Heavy school bags are "dangerous" for children and lead to back problems in later life, a health charity has warned.
Backcare say 120,000 kids could need treatment, with the problem worst among 11 and 12 year olds.
They recommend carrying no more than 10 per cent of your bodyweight on your back but found some kids carry as much as 60 per cent.
The charity want schools to give more advice to pupils.
A spokesman for Backcare said: "More than half of the 4 million children at secondary school will be asked at some stage in their school career to carry excessively heavy loads, endangering their health.
"More than 120,000 will see a doctor before the age of 16 for back pain. And yet, BackCare has discovered, schools receive no advice at all on the impact of heavy school bags on back health.
"Backpacks should be worn on both shoulders with the straps tightened to a snug fit. You should only carry what you really need for that day, and heavier books should be packed closest to the back."