Aiming to bolster student health, Los Angeles Unified and five other major urban school districts announced plans Tuesday to ban the purchase of chickens that have been raised with antibiotics.

The action by the Urban School Food Alliance — which collectively buys more than $552 million of food and supplies annually to feed nearly 2.9 million students daily — will give the food industry a major market incentive to reduce the use of antibiotics in school meals, supporters said.

The widespread presence of antibiotics in food has produced bacteria resistant to it, increasing vulnerability to disease, according to health experts. The Centers for Disease Control has called antibiotic resistance one of the world’s most pressing health problems.

“This is a critical piece of ensuring the safety of our children,” said Mark Izeman of theNatural Resources Defense Council, a New York-based environmental nonprofit that helped develop the alliance plan. “LAUSD and other districts are pushing the entire food industry to move away from chicken and other animals raised with excessive antibiotic use.”

Izeman added that healthful school food is especially critical for the largely low-income students in the six alliance school districts of Los Angeles, New York, Chicago, Miami-Dade, Dallas and Orange County in Orlando, Fla. School meals often provide more than half the food consumed by many of the students daily, he said.

The districts joined forces last year to adopt eco-friendly practices and leverage their collective purchasing power for lower prices and more healthful fare. They have replaced polystyrene and plastic with biodegradable trays and flatware, for instance.

Under the alliance plan, all chicken must be produced with no antibiotics and animal by-products in the feed, be raised on an all-vegetarian diet and treated humanely. If food vendors cannot supply the full volume of chicken under that standard, they will be required to submit a written plan on when they can meet it.

In a separate action Tuesday, the Los Angeles Board of Education unanimously adopted a requirement that chicken purchased for school meals be free of antibiotics and hormones. The poultry rule was added to the district’s 2012 “good food” policy that encouraged more nutritious school meals, sustainable environmental practices, more purchases from small, local farmers, humane treatment of animals and safe and fair working conditions.

Laura Benavidez, L.A. Unified’s deputy food services director, said the requirement would be included in requests for proposals to supply the 2.3 million pounds of chicken purchased annually at a cost of $4.8 million. The school system serves 115 million school meals yearly, second only to New York’s 170 million. It was unclear when schools would begin serving antibiotic-free chicken.

The action marked the district’s latest move toward more healthful school meals. Over the past few years, the district has removed flavored milk from menus, banned soda in vending machines and overhauled school menus to increase fresh produce and reduce salt, added sugars and fat.

“Having antibiotic-free chicken is not a privilege, it’s a right,”  Benavidez said.