I am sure that you can opt out of this.
Both my dcs were weighed. Both flagged as overweight. When I got dds letter (she's 16 now) I was very upset. She was a bit chonky at the time, but did dance for 3 hours a week, swimming probably 90 mins, horse riding probably 2 hours plus competing plus I didn't have a car at the time so we walked everywhere. Any chub she had melted off within 3 months and she shot up in height. Now a skinny 16 yesr old where size 6 in Next is too big.
Ds (7) was and still probably is a bit overweight. I rang the number on the letter and asked for advice. Nurse or whoever it was suggested we exercise more as a family. At the time ds was doing watersports 4 hours a week, riding probably 3 hours a week, on our allotment probably 3 hours a week plus did 10k steps at least 5 times a week on the school run, dog walks and generally running around. He has a healthy appetite and tho he is a veg refuser generally ate well. He is as strong as a bull from the watersports and literally doesn't stop all day.
The number on the scales doesn't always reflect what is happening in that childs life. We are an active family and the kids have always had sports and hobbies. The problem we have is some of those sports are weather dependent so from December to March they may be less active than normal and get a bit heavier during those months. But come May they are out doing stuff pretty much all nights and weekends.
And tbh, if parents are honest they know if their kids are overweight. Weighing them at school won't really change anything for the vast majority. There needs to ve a fundamental change to the diet industry and a fuckton of money invested into grassroots sports and sports and nutrition in school. School dinners leave a lot to be desired, even if they are healthier versions of sponge pudding and custard, it's not good to imply after every meal you get a (proper) pudding. I'd rather they offer a selection of fresh fruit and ditch the puddings altogether.