If you spend more time on something, the result is usually better.

Yet for some reason America doesn't apply it to education. A new National Bureau of Economic Research (NBER) working paper finds that not only does more time in school lead to higher student achievement on internationally standardized tests, but that any diminishing returns from students getting tired or paying less attention are very gradual.

The average American student goes to school for about six and a half hours a day for 180 days a year, or about 1,170 hours. Usually, a good hunk of that day isn't even spent in class. KIPP Academy schools, a type of public charter school, average around 1,700 hours per year and have students that significantly outperform their counterparts at standard public schools.

University of Illinois at Chicago economists Steven Rivkin and Jeffrey Schiman find that better schools and a better classroom environment increase the benefit of the extra time. So a school with poor teachers and a bad environment aren't likely to see much of a gain and won't be able to compensate with extra time. But as schools gets better, extra time becomes more and more valuable.

It might be time we rethought school hours anyway. The reason we have such relatively short school hours and limited days of the year is pure inertia. The summer break and the six to six-and-a-half-hour day came about in the 19th century to accommodate farmers and urban families who wanted to get out of the heat for the summer in an age before air conditioning.

Public schools settled on the current calender in the 1960s, and it's barely shifted since. America has fallen behind other nations in a wide variety of education statistics, yet we haven't changed it.

Some of the top performing countries in the world in education, particularly Japan and South Korea, average over 200 days a year in school, with longer days, and with many students continuing studies after the regular day is over.

It's not just the short days, but a particularly long summer vacation that can hold American students back.

Luckily, about 520,000 students in the U.S. already attend schools with extended hours. Those that aren't on board need to take a look at this chart. The top two charts show how much adding more minutes of instruction each week boosts student scores by grade (left) and in individual subjects (right). The bottom two charts show the effect of adding additional classes in a week: The shorter day was also designed for a world where more people lived in two-parent households and one parent stayed home to take care of the children full time. That's simply not the case anymore, and the way we've designed the school day to end at two or three in the afternoon puts a lot of people, frequently women, at a disadvantage.

Something like a nine-to-five school day would not only give students a better education, but make it easier for their parents to work and raise children without sacrificing the success of either.