Principal Elsa Carmona said her intention is to protect students from their own unhealthful food choices.

"Nutrition wise, it is better for the children to eat at the school," Carmona said. "It's about the nutrition and the excellent quality food that they are able to serve (in the lunchroom). It's milk versus a Coke. But with allergies and any medical issue, of course, we would make an exception."

Carmona said she created the policy six years ago after watching students bring "bottles of soda and flaming hot chips" on field trips for their lunch. Although she would not name any other schools that employ such practices, she said it was fairly common.

A Chicago Public Schools spokeswoman said she could not say how many schools prohibit packed lunches and that decision is left to the judgment of the principals.

"While there is no formal policy, principals use common sense judgment based on their individual school environments," Monique Bond wrote in an email. "In this case, this principal is encouraging the healthier choices and attempting to make an impact that extends beyond the classroom."

Any school that bans homemade lunches also puts more money in the pockets of the district's food provider, Chartwells-Thompson. The federal government pays the district for each free or reduced-price lunch taken, and the caterer receives a set fee from the district per lunch.

At Little Village, most students must take the meals served in the cafeteria or go hungry or both.