I don't have a little one in school yet, but I had a few suggestions.
What about making a milkshake the night before and freezing it solid and taking it out of the freezer to thaw that morning to be ok for lunch? If it worked you could make them on the weekend in a big batch of vanilla and then add different flavorings to each individual one.
Also what about make your own pizza? Pack sauce in a tupperware container, pack the mozz cheese in slices, pack pepperoni, and then they could make it at their desk and would be fun.
Just a few thoughts....don't know if they would really work!