Nutrition at school is essential. And has to be handled in two points.
I remember when I was a child, every year we had "the week of taste" dedicated to food, cuisine and nutrition. We had a lot of activities, tasting stuff, cooking, learning about a balanced diet, etc ... Real cool and it's great souvenirs as well as very educational.
Also the school's cantine were great. It's highly regulated, the meals have to be balance everyday, and offers a lot of variety. Thus if a child isn't used to eat a specific ingredient at home he'll surely meet it for lunch at school. Sometimes children complain and just want a nice steak and french fries but at least they understand how to have an healthy various diet. And when it's the steak and fries day everybody's even more happy :D