Behold the humble spinach pancake. It's cheap, it's simple, it has a good nutritional value and is a childhood favourite of many. Very easy to make as vegan as well and there is a vegan option.
These are probably the all time favourite school lunch in Finland. They are also a very popular ready meal that keeps on being loved by everyone. In school you are allowed to take a stack, or two. This means 10 or 15 if memory serves me right. This depends on the school. Kids like these so much that there has to be a set limit of how many you can take or they run out before everyone gets a share.
Had these for my work lunch today. With lingonberry jam, as is tradition. Back in the day the lingonberries for the school lingonberry jam were collectively picked in autumn, I'm old enough to have done this.
They are made from flour, spinach, egg, water (sometimes milk or partially milk) and salt. You can buy these from any store and everyone eats them, not the worst ready meal imo.
Their cousin is the blood pancake that kids these days don't much like and they aren't offered in school as often, but those are popular still as well. I personally like them very much. They are also eaten with lingonberry jam.
It can be the only thing. These have almost 6g of protein/100g so it is surprisingly filling. There is typically a salad on the side though, maybe something else depending on the school, like cottage cheese.
We also have porridge days in school. It's just oatmeal with a berry soup, a side of bread with some toppings and nothing else.