BOSTON >> Andy Gomez, a ninth-grader at Brighton High School, was not sure why hamburgers and meatballs had disappeared from the cafeteria, but he was not happy about it. “Today I just ate peanut butter and jelly,” he said. “I don’t like the chicken patty.”

The absence of ground beef at lunch last week — at Brighton High and 43 other public schools here — could be explained by a peek into the freezer, where 21 boxes of ground beef products sat, cordoned off from the rest of the meat by a clinical-looking cover of white paper reading “Do not use.”

This is the frozen mass at the center of growing public concern, stoked by news coverage and social media outrage, over so-called pink slime, the low-cost blend of ammonia-treated bits of cow. It turns out that it constitutes some of the ground beef distributed by the U.S. Department of Agriculture through its school lunch program, and that it can be found in at least some grocery store beef, though chains including Kroger, Safeway and Stop & Shop have said they will not sell beef that contains it.

This year, McDonald’s and other fast-food restaurants also said they would stop using the substance, a filler formally known as lean finely textured beef, in their meat products. And on March 15, as an outcry resulted in hundreds of thousands of people signing online petitions, the Agriculture Department announced that next year it would offer schools a ground beef option that does not contain pink slime. Many school districts said they were planning to sign on.

The Miami-Dade school district, one of the nation’s largest, has already said it would opt for pink-slime-free beef, even though it expected it to cost more (exactly how much remained uncertain). State officials in South Carolina said they would procure only the pink-slime-free ground beef once it became available.

But for some school districts — with administrators fielding phone calls from concerned parents and fretting about past food scares — next fall is not soon enough. The Boston school district, among others, has taken the step of purging all ground beef from its menus. Other districts, like the New York City schools, have begun phasing out ground beef containing the additive from their lunchrooms.

Michael Peck, the director of food and nutrition services for the Boston schools, said the district had decided to hold and isolate its entire inventory of ground beef, leaving over 70,000 pounds of beef — worth about $500,000, Peck estimated — confined to a warehouse until the district knows more about what is in it.

“It’s another example of the alteration of our food supply,” said Peck, who is concerned about the use of ammonia hydroxide gas to kill bacteria in the product. “Have we created another unknown safety risk?”

The district will put the meat back into circulation if it finds that it is free of the filler, but like many districts, it is frustrated by the difficulty of determining what does and does not contain lean finely textured beef, which does not have to be listed as an ingredient.

“It does speak to the USDA’s ability to trace,” Peck said. He added that the ground beef would be donated or thrown out if the district found that it contained pink slime.

Rick MacDonald, the assistant director for business affairs at the University of New Hampshire, was working last week to phase in ground beef without “pink slime” for the university’s dining halls, but he said that he and his vendors had trouble finding some filler-free products. His goal, he said, was to have all beef with pink slime gone this week.

“The hamburger patties — we’re trying to find a brand that doesn’t have it,” MacDonald said. “But the problem is, this stuff is so prevalent.”

Still, lean finely textured beef remains approved by the Agriculture Department. The schools’ exodus is grounded less in science than in instinctive revulsion, said Donald W. Schaffner, director of the Center of Advanced Food Technology at Rutgers University.

“I don’t see that there is a scientific or health benefit from the point of microbiology or even toxicology,” Schaffner said of the rush to pull the beef from school menus. “The reason why it’s resonated with people is not so much that it’s unsafe, but the idea that we’re putting ammonia in our food is unpalatable to people.”

Even if removing pink slime quells the queasiness of some parents and school officials, it does not mean much to Fernando Castro, 14, who stood outside Brighton High School on Tuesday, waiting to leave school with some friends.

“I don’t eat school lunch anyway,” he said. “It looks weird.”