The findings – reported in the Archives of Disease in Childhood – said many pupils had “excessively loaded” backpacks.
This was linked to higher levels of back pain in the 1,403 school children taking part in the study.
The research took place at Hospital da Costa in Burela and University Hospital Son Dureta in Palma.
The report’s authors said school children should not carry anything which weighs more than 10% of their body weight.
Measurements were taken from pupils aged 12-17 from 11 schools in Northern Spain. It showed that nearly two thirds of pupils had backpacks which broke the 10% rule.
The weight of the bags was then analysed for back pain, measured as at least 15 days in a year with back pain.
The pupils were split into four groups based on the weight of their bags. Pupils in the group with the heaviest bags were 50% more likely to have reported back pain than in the group with the lightest bags.
The report said back pain was a bigger problem in school girls and that the risk increased with age.
The authors concluded: “The results obtained have strong implications.”
“Many children transport excessively loaded backpacks, an excess which would not be allowed for workers in employment.”
Sean McDougal, from the charity Backcare, said: “The average child in the UK is carrying 15-20% of their body weight to school and back.
“Children are also in the habit of carrying bags over just one shoulder.”
He advises taking to school only what is needed on any given day and ensuring that backpacks are worn over both shoulders.