"I am pretty fat myself and I know exactly why. There is no medical condition that can get around the laws of physics, I'm sick of all this chaval cack about glands etc, complete and utter bowlocks. It's a simple equation."
I'm not sure it's such a simple equation as you are implying. I am not fat, by any reasonable standards, although maybe I'm a little podgier than I'd like after especially the last couple of months. On the other hand, my Mum is. This has been the case for both of us for the entirety of my life. What's also been the case throughout the entirety of my life is that, when it comes to mealtimes, I eat for about three people and Mum eats for about half a person. You absolutely can't accuse her of overeating, nor of being sedentary (she does a lot of gardening, and walked us to school or to the bus stop, and plays or has played badminton, or went swimming for a while, and so on).
Unfortunately the damage was done years and years ago, it seems, and Mum's never been able to lose the weight, in any significant sense. But not because she overeats -- as I say, I can shove enough down my fat gob in one meal that would keep her well-fed for a couple of days, and essentially I've done that day in, day out for 20 years. Yet I've never got fat.
Sometimes, the "simple equation" is screwed in favour of, or against people. I've been lucky, apparently, to have the metabolism needed to avoid putting on weight (if I hadn't I am not sure I would be the world's heaviest man, but presumably I'd give the title-holder a sit-down for his money). My Mum, sadly, hasn't been so lucky.
It does well not to judge people for their body size, until you understand how they got there and what they've done or tried to do about it. Maybe the teacher mentioned in the article was fat due to bad luck rather than for self-inflicted reasons. Or maybe not. But either way, it's a very judgemental attitude to take.
I'm not advocating "fat acceptance" in the sense that we should celebrate obesity, but certainly I'm no fan of being rude about people whose circumstances I don't know.