First, there are differences in the developments of school-age children, what makes it impossible to use the same chairs and tables for all children of the same age. It is not easy to maintain a correct posture while sitting. Second, there is no energy expenditure during sitting, what has negative influence on human digestive system, back, neck, blood circulation, etc. Third, due to the increased spinal loading a sitting child gets tired quickly, loses its attention concentration, etc. And the last but not least – sitting at desks (tables) is a product of the twentieth century, and it is not tenable towards request of public relations in the twenty-first century.
The standing desks have a meaningful effect in moderate activity. So, while learning at the standing desks, children with a standard body weight index burn 15 to 25% of calories, and overweight and those with obesity burn 25 to 35% of calories; The standing postures are more natural for humans and prevents orthopaedic degradation and dysfunctions, including backache, neck pain, etc. The standing desks create a foundation for the environment of movement, and as the researchers say, physical activity is a cognitive candy, that improves attention concentration and cognitive function of a child. 
MET –is the ratio of a person's working metabolic rate relative to their resting metabolic rate. One MET is defined as the energy cost of sitting quietly and is equivalent to a caloric consumption of 1kcal/kg/hour.
What is Moderate-intensity and Vigorous-intensity Physical Activity? Access mode: https://www.who.int/dietphysicalactivity/physical_... www.mdpi.com
and Dr. Mark Benden – Texas A&M Source Access mode: http://standupkids.org