Minimum calorific value. If the mixture of indeterminate origin mince, soya protein and tinned peas/carrot didn't meet the level when taking into account the sugar and fat added to the pudding, they would melt lard into the gravy. If they were feeling generous, watery boiled cabbage to go with it. Often followed by Shepherd's Pie with strange tasting lumps of potato cut out with an ice cream scoop dropped on top of it about 2 days later.
A plate of ravoli with a grating of cheese on top, followed by a piece of sponge cake swimming in custard/rice pudding or semolina pudding with jam.
If it was the last half term of summer, possibly two days where there was salad. A lettuce leaf, tiny wedge of rock hard tomato, a bit of dry cucumber and dusty tasting beetroot cubes.
Once a term there was a slice of pork with two mini roast potatoes underneath a whole ladle of the lard enhanced gravy.
The non pork/vegetarian option was a plate of grated cheese. Sadly, that was more appealing than the standard option.
The most popular dessert was Gypsy Tart with something like Dream Topping on it, followed by a single block of practically liquid vanilla ice cream with the paper embedded into it, again only on one day in the second half of the summer term.
Secondary school was far better - chips (sausages and beans if you wanted them), jacket potatoes with a pat of butter or beans and cheese if you asked really nicely, salad that hadn't been sat in a hot kitchen for hours with identifiable items in it, freshly made cheese or ham rolls with butter and a fair amount of the filling, lasagne made from scratch, fresh fruit, yoghurts, crisps, drinks.
The argument for the minimum calories in primary was that for some children, that would be the only food they got all day. I always thought that if that were the case, they should at least have made it vaguely palatable.