Adults in Scotland do little better, receiving a D-minus for the example they set, with less than one-quarter eating enough fruit and vegetables on a daily basis.
The at-a-glance summary of the habits of Scottish schoolchildren today has been published by experts at Strathclyde University and will allow levels of exercise, obesity and TV addiction to be benchmarked against pupils in 15 other countries.
With more than three-quarters of 11 to 15-year-olds spending more than two hours in front of the television every day, the panel awarded Scotland an F for limiting screen-time. As the number of obese children and teenagers has trebled since 1990, the grade for obesity was F-minus.
John Reilly, professor of physical activity and public health science at Strathclyde University, said: "There is increasing concern that the lifestyles of children are not going in the right direction. We were looking around to paint a picture of where we are in Scotland."
Canada has used a report card system to track unhealthy behaviour among children for almost a decade.
Professor Reilly said there were remarkable similarities between the two nations and early impressions from the other countries due to publish their reports next year were that they also had comparable problems.
Scotland did perform well on two fronts - receiving a B for safe and accessible places for children to enjoy physical activities and a B for national policies and investment intended to encourage healthy lifestyles among the young. These include targets for the number of children who walk to school and participate in sufficient physical exercise at school.
Asked why the other results were so poor given these ratings, Professor Reilly said: "It could be due to a lag of time before good national policies take affect if we are being optimistic or it may be the policies on their own are not enough if there is not enough implementation and monitoring or even consistency.
"These policies have to be maintained over several funding cycles."
The plan is to update the report card in future years to keep track of how well Scotland is progressing.
Professor Geraldine McNeill, from Aberdeen University, also worked on the project. She said: "The card is designed to be a 'one-stop-shop' for information on the major determinants of children's weight and provides benchmarks which will allow us to monitor progress in improving children's activity and diet in the future."
Sport Minister Shona Robison welcomed the report card, saying it recognised the effort and investment provided by the Scottish Government while "clearly laying out the challenges Scotland faces with around three in ten children overweight or obese."
She continued: "Our action plan to cut obesity makes it easier for families to make healthy choices, while our Child Healthy Weight programme supports families whose children are struggling with their weight and want to take action to tackle it.
"In 2012, 70% of children met the recommendation of 60 minutes physical activity a day. We are investing £5.8 m to increase the amount of PE in schools, while latest figures show our Active Schools Network last year gave more children and young people access to over 100 different physical activity opportunities, in more schools, by more coaches and volunteers last year.
"We will also publish a draft Youth Sport Strategy later this year to build on those successes over the next decade."
The report card initiative was funded by the Scottish Universities Insight Institute.