I didn't even think about the emotional side of that story when he told it to me but after seeing the responses here, I went back to ask him about it. According to the teacher:

The kids are all extremely interested in learning the truth about nutrition. This includes the kids who are typically slackers in most if not all other subjects. Some are taking the information home. They greet each other in the hallway with "High Five Low Carb" yells. There was only one thin person in the entire class; his diet was high protein, high fat, low carb. The others are encouraging each other with the new way of life and many are in the process of losing weight. They all seem to be grateful that they can now get to a healthy thin weight and they all seem to be supportive of each other. This is at an inner city Charter school where all the kids come from very poor families. A significant portion of them (at least 25%) are abused; child protective services keeps sending them back. The majority of the rest of them have only a mother who works two jobs; the lucky ones have grandparents at home. Many can't decide which man they like best as the father. Apparently, school is the best place for many of them to be; the best family they have. Maybe the "typical kids who treat each other horribly" are mostly middle and upper class kids that have every video game ever made, that can afford to buy illegal drugs with their allowances, that can drive to the drug dealer homes in the cars given to them, etc.

I was also thinking that when I was a kid, it was a stigma to be overweight. In my kid's classes, the majority of kids are overweight so nobody makes a big deal about it. The ones that get made fun of are the ones with the odd personalities.