For starters, you can use colorful yet informative posters to promote your cause.
I would also suggest you write to the board of education regarding this issue and/or attend one of their meetings (in my town, the board of education hosts monthly meetings where people stand up and ask a question, and the board answers.) You will need to persuade upper management in order to have any sort of action going.
Nowadays, some people don't have a conscience when it comes to food, so the best way is to reduce fats and use healthier ingredients in foods that students love. Pizza is the most obvious example. Vending machines should dispense water and healthy drinks instead of soda. Try to up the quality of food as sometimes the sight of mushy vegetables instantly make some people lose their appetites.
Hope this helps, and good luck!!!