Council figures show the equivalent of 37 bus loads of children in the region start secondary school overweight.
Research shows that one in three children are overweight or obese when leaving primary school.
Guernsey's Healthier Weight Strategy, which has been running since 2023 will be extended until 2027.
The Kent clinic's work is "contributing to national recommendations" for excess weight.
Deep-fried food will be banned and high-sugar items restricted in schools in England under new plans to "overhaul" school dinners.
The scheme is intended to help combat obesity in children and teenagers.
Data from the University of Bristol's Children of the 90s study were used in the research.
Guernsey's Health Improvement Comission says more needs to be done to tackle health inequality.
A measurement programme shows a slight rise in overweight and obese children in two age groups.
