For the first time in more than 15 years, the U.S. Department of Agriculture is raising nutrition standards for school meals, arguing that the "historic" changes will ensure that kids across the nation get healthier meals in their cafeterias. (The rules apply to all schools serving meals subsidized by the federal government.) Here's what you should know:

How will the new rules change school lunches?
They'll set limits on calories: Elementary students will get no more than 650 calories per meal; middle-schoolers will get up to 700 calories; and high-school students will get up to 850. Favorites such as pizza will contain less salt and more whole grains. Breads, buns, cereals, and pastas will have to list whole grain as their No. 1 ingredient. Whole milk is out, low-fat milk is in, and flavored milk must be nonfat. Plus, kids will be getting more servings and bigger portions of fruits and vegetables. Most of these new lunch rules will take effect next school year.

Why the big push now?
It's part of an effort to combat the rising rate of childhood obesity — a third of U.S. kids are now overweight or obese. The 2010 Healthy, Hunger-Free Kids Act directed the USDA to boost school nutrition standards to help get kids into healthy eating habits early. "When we send our kids to school," says First Lady Michelle Obama, "we expect that they won't be eating the kind of fatty, salty, sugary foods that we try to keep them from eating at home."

What kind of impact will this have?
Potentially, it could be tremendous. Nearly 32 million children eat lunch at school every day, and almost 11 million eat breakfast there, too. As a result, American children get between 30 percent and 50 percent of their daily calories in the school cafeteria.

Is everyone pleased?
No. The rules don't go as far as some nutrition advocates wanted. For instance, the USDA had proposed scrapping an existing rule that allowed the tomato paste on pizza to be classified as a vegetable, since striking that classification would have cut the amount of pizza kids could eat. But food companies that sell frozen pizzas to schools objected, and Congress blocked the change, along with another that would have limited servings of potatoes to two a week. School districts also got some rules watered down, saying they would have cost too much. 

How much is this going to cost?
The USDA estimates that the cost of preparing each school lunch will rise by 11 cents under the new rules; the cost of each breakfast will go up 28 cents. The federal government will kick in 6 cents per lunch to help schools meet the standards. Over five years, the total pricetag for the rules will be $3.2 billion. But advocates of the higher standards say they'll pay big dividends by reducing medical bills related to diabetes and other obesity-related conditions. "A healthier population will save billions of dollars in future health care costs," said Dawn Undurraga, staff nutritionist at the Environmental Working Group.

SourcesFox News, Guardian, TIME, USA Today, USDA