In my day (early/mid Seventies at Primary School), you were let into the dining-hall class-by-class as sufficient room cleared from the class before. Everything was cooked on the premises and "dinner money" was 10p per day. It was served by a miserable bunch of Bella Emberg-lookalikes.
Mostly, the dinner consisted of two choices per day from a total repertoire of: mince pie, liver*, meat (beef or lamb - not differentiable) in gravy, toad in the hole, cheese tart. Toad in the hole was my fave - I guess it was the closest to "junk food" that they used to dish up back then. Christmas time would see turkey appear, and the summer would see some salads. I can remember once asking what the meat (in gravy) was, and getting the flat answer: "just meat". The meat (whatever it was) was quite poor quality, with lots of lumps of fat, but then again, what else could you do on 10p per day?
You had potatoes (mashed, roast or boiled - depending on how the cook did them that day). All were edible but they used to "case harden" the roast ones, so they were best avoided. The mash used to have loads of butter in it. You were allowed one veg (perm choice of 2 from: green beans, carrots, peas, mashed swedes, "greens"). The swedes used to literally make me retch, but for some reason you used to force it all down.
"Afters" was usually a sponge pudding and custard (vanilla or a revolting "strawberry" type), or sometime semolina or rice pudding with a spoon of brown sugar in the middle (as kids we always used to stir it round and round till the whole lot went brown).
That's all they ever did. Didn't matter what religion you were (lots of Muslims & Hindus in my school) or whether you were a veggie or not, you ate that or starved (couldn't go outside school). Nobody brought packed lunch that I can remember.
All this was gulped down (I doubt we were ever in there for more that 10 mins, tops), so we could go outside and play again (or listen to one of the dinner ladies read us a story, if wet).
*I always was cross in later life that school liver was cooked to death. Tough, powdery and bitter. Then, as an adult, I had properly-cooked liver in a restaurant and it was *delicious*. I think schools reuined liver forever for lots of people in the UK today.