I have a slightly different take on junk food; personally I am against looking at it as treat, which is how pizza served at school is meant to be taken. It is low quality food and I don't associate it with something special. Once kids get that connection junk=treat it's really hard to change that. Look at all the adults that are having a hard time breaking that training.
For third and fifth grade our son went to a charter school 2 days a week in conjunction with homeschooling 3 days a week. They often seemed to use pizza as a reward for something; the kids did well on the STAR tests let's give them pizza. My son never saw it as a reward. He was used to homemade pizza, his favorite was chevre, feta and Parmesan with rosemary and garlic. He's 14 and he still thinks store bought pizza looks greasy and rubbery.
BTW we've been unschooling our son since the fifth grade and we all love it.