A few years ago I had an argument online about this. It was some TV show where they actually "fat shamed" a character and a bunch of viewers were upset and claiming we should embrace how we look no matter what. My unpopular opinion was, we should certainly not emotionally scar children but this idea that you should be happy being unfit is ridiculous.
You're 100% correct, it starts in school. Phys ed should be required from Grade 1 through graduation. If for no other reason than to keep kids active. As they get older, you can add more advanced education like nutrition, to the phys ed class. I actually took First Aid and CPR in gym class in Grade 11 (the last year it was required). I took Anatomy and Physiology as a separate science class but you could easily roll that into phys ed. But the important thing should be activity, team building, leadership. All important.