or they could serve pizza and veggies?
I think the biggest issue with school lunches is the quality of the healthy food. I don't know how much this changed but when I was in school most lunches came with some vegetables, like green beans, peas, or carrots, and almost no one ate them. It wasn't that we didn't like veggies, or at least that wasn't the only reason, it was because they were disgusting canned veggies that were cooked to an inch of their life and served plain. They smelled terrible, were tasteless at best and nasty at worse, and had the worst texture. I bet that is still what most schools serve, and if it is they aren't going to get the kids to actually eat healthy lunches until they improve the quality, and that takes money.
And no one is going to become obese from a single slice of pizza once a week.