WASHINGTON - The Agriculture Department says 524 schools - out of about 100,000 - have dropped out of the federally subsidized national school lunch program since the government introduced new standards for healthier foods last year.

The new standards have met with grumbling from school nutrition officials who say they are difficult and expensive to follow, conservatives who say the government shouldn't be dictating what kids eat and - unsurprisingly - from some children who say the less-greasy food doesn't taste as good. But USDA says the vast majority of schools are serving healthier food, with some success.

According to USDA data released Monday, around a half-percent of schools have dropped out since last year. Ninety of those 524 schools that have dropped out said specifically that they did so because of the new meal-plan requirements. Most of the rest did not give a reason.

Eighty percent of schools say they have already met the requirements, which went into place at the beginning of the 2012 school year.

"It's important to remember that some schools weren't as close to meeting the new standards, and they may need a little more time for their students to fully embrace the new meals," said Dr. Janey Thornton, the USDA deputy undersecretary in charge of the school meals. She said it is clear that the majority of schools think the new standards are working.

In an effort to stem high childhood obesity levels, the new guidelines set limits on calories and salt, and they phase in more whole grains in federally subsidized meals served in schools' main lunch line. Schools must offer at least one vegetable or fruit per meal and comply with a variety of other specific nutrition requirements. The rules aim to introduce more nutrients to growing kids and also to make old favorites healthier - pizza with low-fat cheese and whole-wheat crust, for example, or baked instead of fried potatoes.

If schools do not follow the rules, or if they drop out, they are not eligible for the federal dollars that reimburse them for free and low-cost meals served to low-income students. That means wealthier schools with fewer needy students are more likely to be able to operate outside of the program.

Some school nutrition officials have said buying the healthier foods put a strain on their budgets. A study by the Pew Charitable Trusts' Kids' Safe and Healthful Foods Project, also released Monday, said that 91 percent of school food officials the group surveyed said they face challenges in putting the standards in place, including problems with food costs and availability, training employees to follow the new guidelines, and a lack of the proper equipment to cook healthier meals.

The group said almost all schools they surveyed had expected to meet the requirements by the end of last year. Even though some schools are still working out the kinks, "It shows that this is certainly doable," said Jessica Donze Black, director of the Pew project, which has lobbied for healthier foods.

Leah Schmidt, president of the School Nutrition Association and director of nutrition programs at a Kansas City, Mo. school district, said any schools that would consider forgoing the federal funds would have to have very few students eating the free and reduced-cost meals.

She said it is to be expected that some schools have met challenges.

"Any time you have something new, you're going to have some growing pains," she said.

Dr. Howell Wechsler, the CEO of the Alliance for a Healthier Generation, a group that is aiming to reduce childhood obesity, said that though some schools are still working to catch up, many have exceeded the standards. The alliance has worked with more than 18,000 schools in all 50 states, and Wechsler says many are thinking of creative ways to encourage healthy eating, like holding walk-a-thons or farmers' markets to raise money instead of bake sales.

He said that many of the schools have reported better academic performance and less student sick days as a result.

"Just about all of the schools that participate with us they say there is a difference," Wechsler said.

As some schools struggled to follow the new guidelines at the beginning of the last school year, USDA relaxed some of the original requirements. In December, the department did away with daily and weekly limits on meats and grains that school nutrition officials said were too hard to follow.

Congress has also had its say on the standards. In 2011, after USDA first proposed them, Congress prohibited the department from limiting potatoes and French fries and allowed school lunchrooms to continue counting tomato paste on pizza as a vegetable.

The school lunch rules apply to federally subsidized lunches served at reduced or no cost to low-income children. Those meals have always been subject to nutritional guidelines because they are partially paid for by the federal government, but the new rules put broader restrictions on what could be served as childhood obesity rates have skyrocketed.

Schoolchildren can still buy additional foods in other parts of the lunchroom and the school. Separate USDA rules to make those foods healthier could go into effect as soon as next year.