They arrive at Hayhurst Elementary by refrigerated truck from Alpenrose Dairy, just a half-mile away in Southwest Portland.
The blue, pink and brown 8-ounce containers of white, low-fat and chocolate "Little Moo" milk are placed in the cafeteria's cooler, from which students pluck them at breakfast and lunchtime every weekday.
Lunchtime is chaotic — four back-to-back 15-minute periods of students filing in and out with their school lunch or home-packed lunch, eating and then tossing their trash before heading out to recess.
Except here, their trash isn't tossed into one garbage can.
The entire school is accustomed to sorting their meal remains using an elaborate recycling station created by volunteer Kendall Kic in 2009.
Seven bins stand side by side in a row at the front and center of the cafeteria.
In a staggered fashion, as students finish eating, they stand up from their tables to pour out their leftover milk. They toss their empty milk cartons and and other tray items into the appropriate bin: durable silverware, energy bar wrappers (collected for the money-back-for-schools program called Terracycle), recyclable plastics such as yogurt cups and juice pouches, and soft plastics such as Ziplocs (which volunteers take to Far West Recycling in Hillsboro).
Near the end of the line, students shake their food scraps into a compost bucket and stack their washable trays.
"I've given up on signs; I just kind of gesture," says Kic, a neighborhood mom whose two sons attended Hayhurst back in 2009.
She still does recycling duty once a week at Hayhurst. She also visits other Portland schools like Forest Park and Robert Gray to help them launch similar efforts.
"The first two weeks it was total chaos" at Hayhurst, Kic recalls. "The line of kids went down the hall. Then they got it, and boom, it was easy."
As students toss their milk cartons into a large Rubbermaid container, she dumps them into another large bin filled with warm water.
She reaches in with her bare hands and opens each carton, dunks it in the water and tosses it to another bin, lined with a mesh bag. (She also cuts open juice boxes, which get recycled curbside, and removes the straws, which go with the plastics to Far West.)
The mesh bag — which holds about 220 milk cartons by the end of the day — gets hung in the school's boiler room overnight to dry. If they're not clean and dry, they can grow mold and no longer be recyclable.
At the end of lunch hour, Kic rinses out each of the bins, helps pour the 4 to 5 gallons of leftover milk down the kitchen sink, and wheels the bins to the side of the cafeteria for the next day.
The next day, the school custodian loads the clean and dry milk cartons into a 32-gallon clear garbage bag designated for recycling in their dumpster.
The school's garbage hauler, Waste Management, transports the five bags of cartons to Far West Recycling in Hillsboro.
Other milk cartons in the Portland area head to other materials recovery facilities, including Pioneer Recycling Services, WestRock and Waste Connections Vancouver.
From Far West, the cartons are packed into 1,500-pound bales, loaded onto trucks and shipped to paper mills in the Pacific Rim.
In their final stage, the Little Moo cartons from Hayhurst and other schools are turned into paper towels, tissue, and office and writing paper, according to Derric Brown, vice president of sustainability for the Carton Council of North America.
"It's a lot of work, but it's been broken down into jobs that can be done quickly," Kic says of the milk cartons' journey. "You get to see the children grow up, and you do save the district money. It's not so difficult any more, because many hands have made it light."