I believe this is a difficult problem for schools. They need to have food that meets the standards (as they are defined), appealing enough to 6 through {age} range to have them eat it, something that can be prepared with relatively low skill demands, and something that can be prepared easily in the quantities needed with the kitchen staff provided.
That really gets down to reheated chicken nuggets, pizza, and other classic school lunches.
The alternative would be to have a school that has a sufficiently large and trained kitchen staff to prepare diverse food, make sure that the food selection that they have meets the requirements (and that the kids aren't just eating the deserts).
I'm recalling back to my school food eating days and the kitchen had four people - two serving, one cooking, one cleaning.
High school had two or three in the cafeteria - and they were constantly putting out the fast food equivalent food items. I can't even remember if there were salads (if there were, I don't think I ever ate them). [Burger, deep fried [fish, shrimp, chicken], French fries] was my lunches for four years.
Though I'm also not entirely sure that schools are to blame for the narrowing of food preference with kids. They don't help, but I'm not entirely sure they are to blame.