Ate public school food in middle america from K-12. Ate a large amount of regrettable things over those years.
I remember much of what impolyt has pictured/ described for his experience with American fare. I had more fruit like apples, grapes, canned peaches and such offered. Greens things were like canned peas, that I still can't think of eating to this day.
I think my favorite substitute for food I couldn't bear to put down my throat were PBJ's made from wonder bread, preservative-laden bulk peanut butter, and grape "flavored" jelly. Must have had a few thousand of these through my school years. The best part was the inedible rubbery yellow cheese stick that would come with it. In 3rd grade we'd take the cheese stick mush it up to a ball and see how high it would bounce. No one dared eat the cheese (mine were yellow though).
I also remember poor bastardized foods labeled as "Mexican" (poor excuse of Tex-Mex) and enjoying them.
I present the hexagonal Tostada (Mexican) Pizza.
My old favorite, the Taco Salad, it was self-serve so I'd load it to the brim. The grade E meat (yes saw it boxes labeled that backside the cafeteria) woud be so greasy it would soak the entire bottom of taco bowl to mush.