Yup. Similar experience here.
We received a letter saying my Son was "Obese". The measurement was taken by weight vs height. He is averagely tall, but really stocky - and as fit as a butchers dog (loads of muscle and runs everywhere). They suggested we change his diet.
We eat only organic veg (veg box delivered is cheaper than the supermarket). Chicken and fish 4 times per week and vegetarian the rest. He has never had a McDonalds/Burger King and we make our own pizzas once a month.
One of his class-mates (who is considered normal) is really, really skinny. We are friends with his parents and know his mother will not allow anything remotely fatty in their diet. When he comes to dinner at ours, he eats like he's not seen food for a week!
My Son's is a happy, healthy, fit young lad. We threw the letter in the bin.