That's easy, Yeti. Go to a big yard sale, or search around on-line. What you want is a Betty Crocker, Better Homes and Gardens, or similar "easy cooking at home" cookbook. Where you've got a choice, focus on the "quick" and "easy": this will usually mean premade shortcuts, such as canned cream of mushroom, tomato, or cheese soup; jello; powdered milk and mashed potatoes; frozen par-boiled vegetables, etc.
The results should be very palatable in a sort of trashy, bland kind of way. If you actually want to reproduce the horrors of a school lunch line, rather than do a little trash-nostalgia, cook everything well in advance using the lowest-price ingredients available and let it all sit covered with foil in a low oven -- not so low that your food rots or anything, but enough so you get that unpleasant warmed-over flavor. Be sure to under-season in terms of herbs and at the same time over-salt. Always use the cheapest margarine in preference to butter.
For a real trip down memory lane, you'll need two assistants. Have the first grunt unpleasantly while dishing the stuff into a plate on a tray, and then slam an over-full cup of Kool-Aid or the like next to it. Now pick it up, and have the other assistant push you from behind unexpectedly and laugh when it all gets mushed up.
Out of interest, why would you want to do this?