According to this link ...
https://www.nhs.uk/live-well/healthy-weight/bmi-calculator/
Your results areL
99th centile
Your child's height and weight suggest they are very overweight
Age: 13
Height: 156cm
Weight: 86kg
Your child falls outside of the healthy weight range of 3rd to the 90th centile.
When a child has excess weight for their age it can lead to health and wellbeing problems like high blood pressure, early signs of type 2 diabetes, low self-esteem and poor confidence as they grow up.
There's lots you can do to help your child become a healthier weight as they grow. Getting your child to eat well and move more is important.
If you wish to discuss your child's weight further speak to your child's school nurse or a GP.