1. Focus on the parents
parents are going to be the ultimate long term driver of this change. their demand for their kids nutrition at school will ultimately lead to change at school. the other two meals (breakfast and dinner) are of course prepared at home. target the parents.
find out what simple differences there are in certain families and duplicate those. more specifically, find a family group that is lean even though genetically everyone else is fat.
figure out what they are doing and repeat it.