Researchers have a prescription for improving the health of America's teens: Get more exercise at school.

Public health experts recommend that kids spend at least 30 minutes of the school day engaged in moderate to vigorous physical activity. That would get them halfway to the goal of exercising for at least an hour each day.

To make that happen, schools would need to devote 7.5 percent of their instructional time to physical fitness. Instead, students are spending a mere 4.8 percent of the school day - or 23.2 minutes - improving their bodies instead of their minds, according to a study published Tuesday in the journal Pediatrics.

"Because adolescents spend so much time at school, even a small increase in the proportion of at-school time spent physically active could lead to meaningful increases in overall physical activity and metabolic health," the study authors wrote.

American teens have a reputation for being among the most sedentary in the world, with only 8 percent getting the recommended 60 minutes of exercise a day.

That exercise deficit sets them up for a host of chronic diseases, including diabetes and heart disease. It also saps their brainpower and causes their grades to suffer, studies show.