In the first study of this age group, Neil Armstrong, professor of health and exercise science, found that physical activity declined as children grew older, although they did more exercise than secondary pupils.
Primary schools should be a key environment for fostering active lifestyles, he said, but warned that imposing programmes of daily physical exercise would prove counterproductive.
The research, commissioned by Persil Funfit, a scheme that promotes fitness in primary schools, found that boys were given more parental encouragement to take part in games than girls and allowed more freedom to display aggressive behaviour. Girls were encouraged to be more dependent and less adventurous.
Professor Armstrong said, for that age group, the curriculum content of PE lessons was more important than the time allocated to them. Teachers should provide a variety of enjoyable activities, he said.
The tenth annual study by Exeter University's schools health education unit of 10 to 15-year-olds found that one-third of 14 to 15-year-olds smoked at least one cigarette a week and nearly half had been offered one or more illegal drugs.
Almost half the 12 to 13-year-olds and two-thirds of the older pupils had drunk some alcohol the previous week.
About one-third felt unsafe outdoors after dark, with 30 per cent of the older boys reporting that they carried some form of weapon.