Half as many adolescents as in 2006 can still buy high-calorie sodas in schools, but other sugary beverages remain easily available onsite, a survey showed.

Another study in the Archives of Pediatrics & Adolescent Medicine last month found one-third of younger students at U.S. elementary schools could still buy sugary drinks, University of Illinois at Chicago researchers said.

The University of Michigan study found sports drinks such as PepsiCo Inc's Gatorade and The Coca-Cola Co's Powerade are still a concern when it comes to older students.

More than half of middle school students (55 percent) and most high school students (83 percent) could still buy the drinks in the 2010-2011 academic year, which ended last June, it showed.

While that is a decline from 72 percent of middle schoolers and 90 percent of high schoolers in 2006-2007, the numbers are still too high, researchers said. Medical experts have said sports drinks should be limited to people doing intense exercise.

"In general, people still perceive sports drinks as a health option for kids," Terry-McElrath said.

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USDA spokeswoman Regan Hopper said the agency could not comment on the study without an extensive review. Its regulation on so-called "competitive" foods was due in December 2011.

Critics blame growing pressure from U.S. lawmakers and beverage industry lobbyists for the delay. Hopper said USDA Secretary Tom Vilsack has asked for more time to review the proposal.

Beverage makers, including Dr. Pepper Snapple Group Inc , point to their voluntary guidelines limiting school offerings that they say have been effective.

USDA's pending rules are supposed to cover food and drinks sold in school vending machines, snack bars, school stores and cafeteria "a la carte" lines. In the meantime, some school districts across the United States have already sought to make voluntarily efforts to push healthier vending machine options.

The study was funded by the Robert Wood Johnson Foundation, a philanthropic organization aimed at improving U.S. health, and has a margin-of-error rate of about plus-or-minus 4 percentage points.