Basically, kids love pizza (it tastes awesome?) and it can be made for dirt cheap, so finding an excuse or loophole to keep it on the menu is what they are doing. Jamie Oliver was doing a good thing (though some of this methods we're a bit overly dramatic) but they weren't interested. The only way to get good food on the menu is to take the crap food off the menu so the kids actually eat it, which obviously isnt going to happen. Crap food is cheaper, its easier and the kids love it.
Pizza really is not healthy, in moderation sure, but all that bread even at the best of times is not something you should eat often, thats before you get into the cheapy crap low quality saturated fat filled salty cheese bread.
When I was in school we had a load of junk food on the menu, I used to live off of pasties and hotdogs (though with real sausage, not like the american type), we had big inhouse baked cookies, coca cola vending machines and chips were probably the most sold food, all of that is gone now though, you're basically forced into eating atleast half decent here in the UK schools.