Maybe it varies depending on the school. My son attends a small, village school.
When they started in reception, the teacher went to lunch with them at first and kept an eye on who ate what/who wasn't eating etc. She would feedback concerns to parents. My son has always been very tall for his age (but skinny - absolutely perfect bmi, confirmed by the GP) and was eating everything on his plate but getting hungry later. So, the dinner lady started to give him a bit more - problem solved.
He's just finished Y2 and still gets on well with school dinners but the staff know the children well.
All that said, he still needs a snack when he gets home! Afternoon learning must make him hungry!
Typically, his meals look like this:
Breakfast: cereal (e.g. 2 weetabix, cornflakes- no sugar) with semi skimmed milk. Drink of milk. Toast/fruit.
Snack: Fruit/breadsticks at school plus milk (though it will be water next year in Y3).
School dinner: varies but he might have macaroni cheese, broccoli, potatoes and biscuit for afters.
Post school snack: apple and cheese/breadsticks/toast - something like that.
Tea: Varies but probably something like fish, veg and potatoes or spaghetti bolognaise or sausages, mash, veg.
He usually has fruit/cheese+crackers after.
Supper: drink of milk and a banana at bed time.
He has a very good appetite but remains skinny! I honestly don't know where he puts it! He is very active though.
You could mention it politely to the school or send packed lunch.