Obesity experts say that just increasing the time that kids spend in exercise will not guarantee that they will get more action and thus reduce weight. Economist John Cawley of Cornell University and colleagues found in a new study that government data showed the number of students taking part in physical education decreased from 42% in 1991 to 33% in 2005.

They also examined data on 37,000 teens in grades nine through 12. All students had taken part in government surveys in 1999, 2001 and 2003. They found that many students were not getting the exercise despite increased hours. "Some schools are ignoring the laws and not meeting the state requirements," Cawley said.

"The real risk here is that states may increase the time requirements, think they've addressed the problem of childhood obesity and may move on to other priorities," he added. The paper is published in the latest issue of Education Next.