State laws that ban soda in schools — but not other sweetened beverages — have virtually no impact on the amount of sugary drinks middle school students buy and consume at school, a new study shows.

The study, which looked at thousands of public school students across 40 states, found that removing soda from cafeterias and school vending machines only prompted students to buy sports drinks, sweetened fruit drinks and other sugar-laden beverages instead. In states that banned only soda, students bought and consumed sugary drinks just as frequently at school as their peers in states where there were no bans at all.

The study is among the first to directly examine the extent to which state policies on soda in schools influence students’ behavior. With obesity on the rise and teenagers getting about 15 percent of their daily calories from beverages, health groups like the Institute of Medicine have pushed for the removal of all sweetened beverages from schools, and some states have put in place all-out bans on sweetened drinks. California, for example, became the first state to ban the sale of soft drinks in grade schools, in 2003, and one city, Boston, moved earlier this year to forbid the sale and promotion of sugar-sweetened beverages and sodas on all city property.

But the sale of soft drinks in schools has become a lucrative revenue source for many school districts, and a number of states have been reluctant to eliminate them from schools. Some states have instituted only partial bans that remove sodas from schools but not Snapple, Gatorade and other sugary drinks.

In the latest study, published this week in The Archives of Pediatrics and Adolescent Medicine, researchers looked at soda and sweetened beverage consumption among roughly 7,000 fifth and eighth grade students across 40 states. The students were followed between 2004 and 2007, a time when many states and school districts were beginning to enact bans.

The researchers found that the proportion of students who purchased sugary drinks at schools in states that removed only soda — 28.9 percent — was similar to the proportion in states that had no bans against any sugary drinks at all — 26 percent. Both groups  also reported having identical levels of access to sugary beverages at school: about 67 percent in each category.

“I think definitely the biggest message is that laws need to be comprehensive to have any positive effect at all,” said Daniel R. Taber, an author of the study and postdoctoral research associate at the University of Illinois at Chicago. “The most unequivocal finding was that laws that focus on soda are just not getting it done. If you really want to create a healthier school environment, you need more comprehensive laws.”

But the study, which was funded by the National Institutes of Health and the Robert Wood Johnson Foundation, had another surprising finding. In states with policies banning all sugary drinks in schools, students had less access to them on campus, but their overall consumption of the drinks did not fall, suggesting they were getting the drinks elsewhere. Dr. Taber said that finding indicated that removing sugary drinks from school campuses, while effective, could only go so far. He said more initiatives that target sweetened beverage consumption outside of schools — like beverage taxes and regulations on their marketing to children — were needed.

“The laws did exactly what they were designed to do,” he said. “They were designed to reduce kids’ access to sweetened beverages in schools, but you can’t expect schools to do it all on their own.”

Dr. Taber said it made sense for some school districts to focus on soda at first, since it accounts for about two thirds of the calories teenagers get from sugary drinks. But that could also give some students the wrong idea about which drinks are best.

“Soda is definitely the most popular choice among kids,” he said. “But there’s a lot of misconceptions about which beverages are healthy. Many kids think beverages like Gatorade are a healthy alternative to soda.”