WASHINGTON — Sugary soft drinks accounted for less than a quarter of the beverages sold in schools last year, demonstrating that a voluntary transition toward healthier drinks is working, according to the beverage industry.

The current school year will mark the final year of a three-year, voluntary program designed to lower the calories and portions offered to students through school vending machines.

Overall, an analysis of the beverages offered the latest school year showed a significant change since 2004. Part of that change stems from the program's guidelines, but many states also banned the sale of soft drinks on campuses. Nondiet soft drinks have fallen from about 40 percent of the drinks that bottling companies shipped to schools to about a quarter of the mix.

Bottles of water now take up the largest slice of drinks offered in schools. Bottled waters represented 27.6 percent of the product mix last year, versus 22 percent the year before and 13 percent in 2004.

"We recognize that schools are unique places and we're doing our part to help students understand the importance of balancing calories burned with calories consumed," said Susan Neely, president and chief executive officer of the American Beverage Association.

Health officials long have expressed concern that schools contributed to rising obesity rates because campus vending machines sold high-calorie and high-sugar snacks and drinks.

In May 2006, leading beverage companies agreed to work with the American Heart Association and the William J. Clinton Foundation in accelerating a shift toward healthier drinks with fewer calories and smaller portions. Their agreement included the removal of full-calorie soft drinks by the 2009-10 school year.

The voluntary guidelines state that elementary and middle school vending machines will only sell 100 percent juice, low-fat milk and bottled water. For high schools, diet sodas, sports drinks, waters and teas can be added to the mix.

Former President Clinton said he was happy with the results for Year 2 of the agreement. He noted that about 80 percent of schools in contracts with the leading beverage companies were in compliance with the guidelines. That surpassed the goal of 75 percent compliance.

"They did better than they said they'd do, and the results they delivered surpassed where we thought we would be," Clinton said during a news conference last week in New York.

The progress report presented by the beverage industry showed a drop in juice drinks offered in schools as well as increases in sport drinks and diet sodas. The major swing toward lower-calorie beverages resulted in a 58 percent decrease in total beverage calories shipped to schools between 2004 and the 2007-08 school year.