Chicago ? Cartons of milk still anchor school lunches, but the milk inside them is changing as concerns mount about childhood obesity and nutrition.

Some schools are adding to the selection of sweetened varieties in an effort to boost students’ calcium intake, while other schools ban flavored milks. Chicago Public Schools stopped offering whole milk out of concern for the added calories and fat. Organic milk is edging into some lunch lines, while others now offer soy milk.

“If there doesn’t seem to be a set standard, it’s because there is none. School wellness policies are individual to districts,” said registered dietitian Melissa Joy Dobbins of the Midwest Dairy Council. But when it comes to what kind of milk schools should offer these days, she said, “There’s a lot of chatter.”

The focus on milk follows other recent efforts to improve school nutrition, including banning soda, limiting transfats in lunches, purging junk food from vending machines and banishing cupcakes from classroom parties.

The milk debate comes amid concerns that dairy consumption is waning among older children who have more beverage choices, from flavored water to energy drinks. Nine of every 10 preteen girls fall short of the federally recommended three calcium servings a day, according to the National Health and Nutrition Examination Survey. For boys, the estimate is 7 of 10.

This is troubling because bone density peaks during adolescence. Calcium is vital to bone strength. So the shift away from milk — the main calcium source for 80 percent of children, according to the American Dietetic Association –could portend future health problems.

“The calcium bible says, basically, that by the time you’re 19 or 20, you’ve achieved your peak bone mass. The assumption there being the better bones you have in the beginning, the better bones you’ll have when you’re 65,” said Dr. Frank Greer of Madison, Wis., who chairs the nutrition committee of the American Academy of Pediatrics.

One way schools seek to compete with other drink choices is to offer a greater selection of flavors. But the addition of milk flavorings raises the specter of another health concern: childhood obesity.

A half-pint of low-fat chocolate milk has 3 teaspoons of added sugar — the same amount found in a packet of cinnamon oatmeal or half a banana, for example, the Midwest Dairy Council reports.

Those extra 75 calories raise a concern, given that surveys compiled by the Centers for Disease Control and Prevention have found that 17 percent of school-age children are obese. Others say it’s the lesser of two evils.

“We would prefer children drink unflavored milk, but it’s better for them to drink flavored milk than no milk at all,” said spokeswoman Jean Daniel of the federal Food, Nutrition and Consumer Services agency.

“If I don’t have it here, if I don’t have a selection, the high school kids will go off campus and get it,” said Micheline Piekarski, food service director at Oak Park and River Forest High School.