The compostable plates are the first test of the alliance’s thesis. This week, the New York City Education Department will open sealed bids to supply the roughly 850,000 plates it needs each day for breakfast and lunch programs in about 1,200 schools. New York is running a pilot program, like Miami’s, in four schools, with 30 more expected to join this month.

If a winning bidder is chosen, the other alliance members will be able to piggyback on the contract, placing their own orders without having to navigate a separate bidding process. The call for bids names all six districts and says they must all be allowed to place orders at the same price.

The alliance’s next target is healthier food. It is already looking at potential suppliers of antibiotic-free chicken. School officials say possible future initiatives include sustainable tableware, pesticide-free fruit and goods with less packaging waste.

The direct benefits of these efforts may not always be obvious, or even noticeable. To a child, antibiotic-free chicken tastes like any other chicken. And even a huge purchase by the alliance would have little effect on farmers’ preferences for giving animals antibiotics, much less on the danger the practice poses: spawning new classes of antibiotic-resistant bacteria.

But short-term environmental and health benefits are not the only goals, said Eric Goldstein, the chief executive of school support services in New York City. Using recyclable plates or serving healthier chicken sets an example that students may carry into adulthood, he said, and that other school systems may come to see as a standard.

“It sounds corny,” Mr. Goldstein said, “but we all believe in this.”

The six districts banded together in July 2012 at a school-nutrition conference in Denver. They received a lift later last year when the Natural Resources Defense Council, a national advocacy group with a history of pressing governments for environment-friendly changes, met with Mr. Goldstein and other New York school executives to talk about recycling and healthier food.