What does moderate to vigorous physical activity mean for kids?  Philly.com  by Rachel DeHaven, Physical Activity Specialist, Children’s Hospital of Philadelphia

Simply put, moderate and vigorous are terms to explain how hard your body is working when doing that particular activity. Exercise physiologists measure activity in metabolic equivalents, or METS. One MET is define as the energy used at rest.

Even Small Bursts of Exercise Can Make Kids Healthier. TIME  by Jeffrey Kluger  

In secondary students as little as 20 minutes of activity three times a week could go a long way.