T.F.
The elementary school I sub at has items on ala carte, however, most parents have restrictions on the account code and the children can't get it. Many parents opt to pack the children's lunches and avoid the lunch line altogether.
I believe it teaching good eating habits begins at home and by parents modeling those habits. I don't think forbidding certain things will do anything except make someone want it more.
Moderation is the key. I know my 16 yr old will eat fruit anyday over chips or candy etc but it is how she was raised.