Well, lots of people die of cancer. When they do, it's likely they will have cachexia, wasting away. If they are in the death figures, well of course a low BMI will look unsafe. Just because people have a low BMI when they die, it doesn't mean the BMI is the cause.
Ditto for the high BMI.
Back to the kids, I saw seven teenagers waiting for their school bus this morning. None of them looked obese, or even fat. Unless the new standard of obesity is not obvious to the casual viewer. By my calcs, the chances of no child in seven being obese, when the obesity rate is 24%, is about 15%. Maybe further research is needed.