Parents are to blame for making nine in 10 school bags so heavy students risk injuring their backs, the council says. The council said heavy textbooks were no longer the problem in school bags. Pupils were being weighed down by sports gear, spare clothing and food containers - much of which they did not need on the day they were checked. Council spokesman Dr Michael Tsui said the council had weighed the contents of school bags belonging to 500 Primary Three to Secondary Two students. The results were 'shocking and disappointing' given years of publicity about the damage done to children's spines by regularly carrying heavy bags. Students should not carry a bag weighing more than 10 per cent of their own body weight. But the council's weigh-in found 90 per cent of primary students and nearly 70 per cent of secondary pupils were overloaded. In the worst case, a Primary Five boy was carrying a school bag weighing more than seven kilograms - nearly a third of the pupil's body weight. 'It's terrible,' Dr Tsui said. 'Parents must be responsible. It's up to them to make sure their children's bags are packed every day, not just once a week.'