From Frank Chalk's memoir of teaching in a British school ("It's Your Time You're Wasting"):
Exercise is a major key to solving behavioural problems, I think, as I watch them hare around the field having fun. Nowadays, they do far less physical activity than the children of the past. Labour-saving devices are everywhere to enable more time to be spent in front of the television. Kids are driven to school because parents have got it into their heads that child molesters lurk behind every hedge.
But our bodies have not evolved to be sedentary and need regular exercise; kids have loads of energy and they can either use it up through running around or they can use it up in your lesson, causing mayhem. Young men, especially, need an outlet for their natural physical aggression to prevent it bubbling over at an inappropriate time.
Ask any teacher who has taken kids on a walking holiday, or has been on a school ski trip abroad; they are no trouble after the first three days. They burn it all off during the day and are pleasantly tired in the evening. I've seen kids with the most awful records and every fashionable acronym complaint under the sun sit still and listen for the first time in years, simply because they doing what their bodies have evolved to do, often for the first time in their lives. When they return home they immediately revert to their former behaviour.