I noticed the same thing at my daughter's musical Wednesday, and that was at the elementary school.
We went to a Friday night fundraiser, and I noticed that the parents of the obese children were not necessarily obese themselves. They were letting their kids eat lots of junk food. Of course, it doesn't help that the only food provided was junk food. That's where the real problem is, if the only food provided is junk, kids are going to eat it. It's everywhere, and it's normal food for them.
There was a lot of opposition when a neighboring town's school district decided to start serving healthy foods. That meaning a salad bar and grilled chicken instead of chicken nuggets, canned corn, and tater tots. The lunchroom workers were opposed to it, because it's harder to serve fresh food rather than prepared food. The school superintendent himself was opposed to the idea and was quoted as saying "you can put it out there, but the kids wont eat that". Well, guess what? They did. And they liked it.
I was going to get on the school lunch committee, until I learned that they still had to stick to the USDA food pyramid anyway. What's the point of that when the Food Pyramid doles out over a cup of sugar per day? The only way out of that is to opt out of Federal funding, and that's not really an option for most public schools. Healthful foods are expensive.