July 9 was the first day of school for 26,000 students in North Carolina's Wake County Schools on a year-round calendar.

But year-round schools, which were once considered options to improve student performance and reduce classroom overcrowding, have proven to be a mixed bag. Some school districts that have adopted the extended calendars, like Las Vegas and Salt Lake City, have turned back to traditional academic calendars.

Extended school year supporters believe that it keeps kids from falling behind academically by avoiding summer slump, and keeps troubled kids off the streets. Research has also shown that students in high-needs districts and students with special needs tend to do better in schools with extended calendars.

But that doesn't apply to everyone, Rick Hess, director of education policy at the American Enterprise Institute, tells Fox News.

"I think extending the school year for everybody would be a really bad idea," he said Tuesday. "We want to extend the school year for kids for whom it would benefit them and for kids who are attending schools where we're confident the time's going be used well and it's going to be used effectively."

So we shouldn't suggest creating a federal policy around a longer school calendar?

"I think that would be a horrendous mistake, and I think statewide policies would be a horrendous mistake," Hess said.

While Denver Public Schools' Manual High School started off the district's longest school year last month, Hess says summer experiences like travel and camps are critical. Manual has added 39 extra days to its calendar, as well as an additional hour to each day.

Hess notes that children from less educated, low income families are more likely to experience summer learning loss, but mandating a longer calendar for all students will not prove beneficial.

"Even when children start school at age six in more or less the same space, kids from low income or less educated families are a few years behind by the time they get to high school," Hess said. "I think we owe it to those kids to do something about it."

Watch the video above to see what else Hess and Peter Gray, research professor of psychology at Boston College, have to say.