INDEPENDENT action is being taken by primary school principals to tackle the weighty problem of pupils' bags. Despite long-term concern that school bags are too heavy and possibly harmful to health, thousands of children still travel to school laden down with books. The Government is establishing a committee to seek medical advice on how heavy bags can be without endangering pupils' health, and whether publishers can help by producing smaller and lighter textbooks. Guidelines will be issued to schools when the study is completed, but principals are about to conduct their own survey to identify ways of easing the problem. Orthopaedic experts believe bags weighing more than one-tenth of a child's body weight can be damaging, but every day youngsters struggle to school carrying far heavier bags. Textbooks, exercise books, pens, pencils and snacks, sometimes adding up to more than five kilograms, have to be lugged to and from school by the children, who are not provided with lockers. Members of the Subsidised Primary Schools Council have devised a questionnaire for principals to give their views on how the problem might be alleviated. The principal of the Building Contractors Association School in Tin Hau, Fung Chiu-wing, said he was concerned that pupils were over-burdening themselves with books. Primary One pupil Li Sing-fu's bag, for example, weighed in at almost six kg. It contained 15 textbooks, 25 exercise books, dozens of work sheets, a newspaper to use in a painting class, as well as pencils and a fair amount of rubbish. Many of the exercise books were finished, while others had not been started. But Sing-fu did not mind the load. ''It is better to carry everything, then you know you have not forgotten any books,'' he said. His classmate, seven-year-old Vicky Chan Wai-kuen, came to school burdened with a bag which tipped the scales at about 5.5 kg. She weighs 23 kg.