As Chicago teachers continue to strike, one issue up for debate is whether the city's schools should add more learning time, with longer days or more days in school each year.
One reason the question continues to be so contentious is that there's very little hard evidence to back up theories on either side, said Erika Patall, an educational psychologist at the University of Texas, Austin.
Two years ago, Patall and colleagues attempted to do a systematic meta-analysis on the issue, but they could only find 15 studies that used actual data to address the question of how extended school time affected educational outcomes. Each study was so different from all of the others that comparing them with each other was impossible.
Her final conclusion was that adding time to the school calendar probably has no negative effects and might help slightly, especially for at-risk youth with few resources and limited access to extracurricular activities.
"When the Chicago thing happened, I got calls from people on both sides using my article to support their claims, and the reason why they could do that is it's pretty ambiguous," Patall said. "The bottom line is that we don't have a really good idea about how extending the school day or year really plays out because there's not good research. So it's mostly a theoretical debate based on personal beliefs.
The length of the school year varies from state to state and district to district, but it averages about 180 days that are six and a half hours long -- numbers that were established in the 1920s, when school attendance became mandatory.
The school calendar was originally designed to accommodate the demands of farms and factories, said Jennifer Davis, president of the National Center on Time & Learning, an organization that advocates for expanded learning time. Summer breaks allowed some families to escape the oppressive heat of the city during the hottest months.
About a decade ago, the charter school movement began to break free from standard ideas about school schedules, Davis said. In 2009, Obama drew yet more attention to the issue with a speech that advocated for extended hours.
"We can no longer afford an academic calendar designed when America was a nation of farmers who needed their children at home plowing the land at the end of each day," Obama said in his speech. "That calendar may have once made sense, but today, it puts us at a competitive disadvantage. Our children spend over a month less in school than children in South Korea. That is no way to prepare them for a 21st Century economy."
Advocates argue that more time in school should, among other benefits, boost academic achievement, allow for more opportunities to pursue experiential learning, and contribute to deeper relationships between teachers and students.
Opponents argue that extra time has the potential to exacerbate fatigue and boredom while reducing opportunities for the kind of informal learning that happens outside of the classroom in extracurricular activities, jobs and free time.
There is a clear socioeconomic divide between proponents on either side. Parents with fewer resources generally want more school time for their kids. More advantaged parents tend to support business as usual.
The limited available research supports that divide. All kids lose ground in mathematics during the summer break, Davis said. But the slip in reading is most pronounced in kids who live in poverty. Studies that support more time in school tend to show the greatest benefits in the most disadvantaged kids.
For now, though, most of the studies that support more school time show that there's a relationship between extra time and better outcomes, but they can't necessarily prove that extra time is what causes the benefits.
Given the difference in opinion and the socioeconomic divide, Patall supports the idea of giving parents a choice between schools that offer shorter or longer schedules.
No matter how the debate ends up affecting policy, most experts agree that more time is only better if it is used well.
"A random act of adding an hour or two hours to the day that is not done thoughtfully and not done with a focus on the educational needs of those students, the quality of teaching, and other elements of a strong school, don't tend to show the same educational impact," Davis said. "Our studies and research document what high-performing, high-poverty, expanded learning time schools do. They use every minute strategically."