you say "it's just lunch" as if lunch isn't important.
Do a poll online. How many people sent their children to school with the expectation that because the child would be getting a hot lunch, he/she would only need a snack tea?
And school dinners were designed to be the main meal of the day, and certainly for parents on benefits, they ARE supposed to be the child's main meal of the day! They are supposed to provide a good whack of fat, protein, iron, calcium, fibre, and various micronutrients, whereas all they seem to ACTUALLY provide is processed wheat in various forms.
Ds1 frequently eats more bread with his free school dinner than he actually gets given for a free school dinner. This meal is supposed to be an adequate 33% of his diet - if I fed him like that for the other two meals, the ones I am in charge of, he'd be malnourished. If three of a certain quality of meal per day will make a child malnourished, it shouldn't be a meal option. My child should not be able to choose to have rice with mashed potatoes (and nothing else because the meat has run out and the vegetable is sweetcorn for the fourth day running), , with a fairy cake for pudding. It should not even be available to him in that format.