There is Pizza, and then there is “pizza”. Just as there is tomato sauce and “tomato sauce”.
For example, for a classic Pizza Margherita, the dough is just flour, water, yeast, salt. like bread. the sauce is just crushed tomatoes, a pinch of salt, and some basil, and you only use a small amount to coat the pizza. Mozarella is a lower fat cheese. The end result is something that you can call healthy.
Which is of course very different from industrialised “pizza”, which is loaded with salt, fat, sugar, and whatnot.
When you pile up your sugar filled pizza base with basically ketchup, tons of meat, fatty cheese, and more cheese, then the result is no longer healthy.
I very much doubt school lunches serve real pizza. They are going for the frozen junk pizza, because it is cheap and convenient.