Even though teens get more exercise at school than anywhere else, it still isn't enough to meet minimum daily activity levels recommended for good physical and mental health, a U.S. study suggests.

Children and teens should get at least 60 minutes of moderate to vigorous activity every day, the World Health Organization recommends. But in the U.S., only an estimated 8 percent of youth meet this standard, researchers report in the journal Pediatrics.

Schools, where teens spend far more waking hours than anywhere else, appear to be a big part of the problem.

On school days, adolescents got an average of just 23 minutes of physical activity at school, and this accounted for more than half of the 42-minute daily total, the study found.

Taking weekends into account, teens were even less active over the course of the entire week, averaging only about 39 minutes of activity daily with the majority of exercise still happening at school.

"We knew that schools were a major source of physical activity for kids, but we were surprised that kids spent only 4.8 percent of their time at school physically active," said lead study author Jordan Carlson of Children's Mercy Hospital and the University of Missouri in Kansas City.

"Kids have a natural instinct to move around, and schools can support this by providing more opportunities for students to be active, such as by incorporating physical activity in the classroom," Carlson added by email.

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Plenty of previous research has found many teenagers tend to be too sedentary and documented a declining focus on physical education in U.S. schools. The current study set out to shed new light on where adolescents get whatever limited exercise does occur.

For the current study, researchers asked more than 900 students aged 12 to 16 years living in the metropolitan areas around Washington, D.C. and Seattle to wear activity trackers that mapped when and where they exercised over the course of one week, including both school days and weekends.

Carlson and colleagues analyzed data from a subset of 550 teens who wore the trackers for at least one school day and one weekend day. Most spent a full week wearing the trackers.

On average, the teens spent 42 percent of waking time at school during the week, followed by home, which accounted for 28 percent of their time. They also spent about 13 percent of their time near home and an additional 4 percent of time close to school.

Teens passed the majority of their physically active time on school days either near home or close to school, the study found. This probably included travel to and from school as well as recreational activities in the neighborhood, the researchers conclude.

It's possible that some of exercise that happened at school didn't occur during the regular school day, but instead happened as part of after-school sports or other programs, the researchers note.

The study also lacked specifics about the locations outside of home and school where students got some of their exercise, the authors concede.

Even so, the findings suggest that interventions designed to increase exercise opportunities at school may help students become more physically active, said Penny Gordon-Larsen, a public health researcher at the University of North Carolina Chapel Hill.

"Physical activity provides multiple health benefits, including physical, cognitive and psychosocial health benefits," Gordon-Larsen, who wasn't involved in the study, said by email. "Low physical activity puts teens at current and future risk for obesity and cardiometabolic diseases."

It may also be bad for their grades, noted Maureen Dobbins, a nursing researcher at McMaster University in Hamilton, Ontario, who wasn't involved in the study.

"The evidence demonstrates a clear link between physical activity and student achievement," Dobbins said by email. "So while math, science, language etc. are important, time spent engaged in moderate to vigorous physical activity is an important strategy in helping teens achieve academic success."