Roughly 40 percent of the city’s elementary and middle school students are overweight or obese, according to the Education Department. The department also found a correlation between student health and performance on standardized tests, according to a survey it released in July.

The previous regulations limited sales to once a month and allowed them at any time during that day, but they were loosely enforced. Officials say they will do more to monitor the new regulations.

“We have an undeniable problem in the city, state and the country with obesity ,” said Eric Goldstein, the chief of the office of school support services. “During the school day, we have to focus on what is healthy for the mind and the body.”

Unsurprisingly, the rationale is getting a cool reception among students. At Fiorello H. La Guardia High School on the Upper West Side, students are used to having bake sales several times a month. Now, Yardain Amron, a sophomore basketball player, laments that his team will not be able to raise money for a new scoreboard.

Another La Guardia student, Eli Salamon-Abrams, 14, said that when the soccer team held a bake sale in May, his blueberry muffins sold out in 15 minutes. He said of the ban: “I think it’s kind of pointless. I mean, why can’t we have bake sales?”

The new policy also requires that vending machines, which generate millions of dollars for school sports, be supplied with snacks such as reduced-fat Baked Doritos and low-sugar granola bars. A new vending machine contract is expected to be approved on Wednesday by the Panel for Educational Policy , the school oversight board. Student stores will be able to sell only approved snacks bought from the new vendor, rather than obtain the food themselves, as they once did.

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Principals are expected to enforce the new rules. “Noncompliance may result in adverse impact on the principal’s compliance performance rating,” the policy states.

With the changes, school administrators and teachers who oversee student clubs are laboring to come up with other easy ways to raise money, particularly at a time when school budgets are being cut.

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John Sommers, the assistant principal of organization at La Guardia, said that all fund-raisers using food were on hold for now.

He said teachers had encouraged students for years to be careful with what they sold. “There was never any cotton candy or something like that, and there weren’t sales all the time,” he said. “But they are definitely a way kids count on to get money.”

A typical weekday sale, he said, could bring in about $500 in profit. “If they wanted to buy some uniforms or go on a trip, that was enough,” he added.

Mr. Sommers said he was trying to figure out other ways for students to raise money, perhaps by selling T-shirts or key chains. (All of which are decidedly more expensive to produce than a box of brownies.)

Department officials are suggesting that teams use walk-a-thons and similar activities as a way of raising money and doing something active.

For all the changes, there is much the regulations do not address. For instance, there are no stipulations of what kind of treats students may bring to class, so birthday cupcakes appear to be safe. Snack bars of any kind are permitted at after-school sporting events, a prime time for cheese-laden nachos and fatty hot dogs.

Schools around the United States , including throughout California , have banned bake sales or put a limit on the sugar and fat content of the goodies. But New York’s regulations are among the strictest in the country, said Howell Wechsler, the director of the division of adolescent and school health at the Centers for Disease Control and Prevention .

“There are more schools that are making more changes in what is available for kids at school,” said Dr. Wechsler, who has studied nutrition policies at schools nationwide. “Schools are supposed to be a place where we establish a model environment, and the last thing kids need is an extra source of pointless calories .”