Did kids always have to go to school for most of the year?

Nope. For a long time they didn't have to go to school at all. Then in the mid-1800s, Horace Mann, secretary of education for Massachusetts, persuaded the state to require kids to attend school for at least 12 weeks. Other states began doing the same thing. Mann and other educators wanted to give everyone an equal chance to learn, realizing that more people needed to be educated to help the country grow. Adults also wanted to protect kids who were being forced to work.

How did we jump to 180 days?

We didn't jump; we got there slowly. The U.S. Department of Education says that in 1869-70, the average length of the school term was 132 days. (But kids played hooky a lot: The average number of days each student attended was only 78.) By 1900, the average school term was 144 days. By 1950, the school year had reached 178. (And by then, it was harder to skip school: On average, kids attended 161 days each year.)

Why did kids get the summer months off?

In earlier times, schools followed a planter's schedule, allowing children to work at home during the busiest summer months when the harvest was in. That pattern held even after many farms disappeared and even in places where nobody planted. In some areas it was simply too hot in summer to have school. And many educators believed that children needed a break from school to relax and play.

So who gets to decide how long we have to be in school?

State governments make that call. Local districts, though, set the start and end dates.

But why 180? Why not 179, or 183?

Actually, some states don't require 180 days, and the number of hours of schooling isn't always the same. According to the Education Commission of States, kids in Alabama, Vermont and Wyoming have to go to school for a minimum of 175 days; in Hawaii, 184. Connecticut requires 180 days, with a total of 900 hours, while Delaware requires 180 days but 1,060 hours. Minnesota is the only state that allows its school districts to decide. Most states (including Maryland and Virginia) and the District settled on 180 days, a number easily divided into four nine-week periods of 45 days each, or two semesters of 90 days.

So what happens if schools can't be in session for 180 days?

Sometimes, like this year when there have been so many snow days, kids wind up with fewer than 180 days. The Maryland Board of Education said that schools do not have to make up two of the days that were missed because of the big snowstorm. But students aren't off the hook completely. Most school districts will be adding days to the end of the school year, canceling days off for teacher work days or shortening spring breaks to make up for time missed.

Is 180 days also the magic number for other countries?

The French use a 180-day schedule, but plenty of kids around the world go to school for much longer than Americans. Students in Japan, South Korea and Taiwan, for example, are in class for at least 220 days. Researchers say the average around the world is 200 days.

Will Americans increase the number of days?

Some states probably will over time. Plenty of educators think 180 days is not enough to teach kids everything they need to know. Some U.S. schools already require 200 or more days of schooling. And David Berliner, a professor of education at Arizona State University, predicts that one day the 200-day school year will be the rule in America.

Now that mountains of snow have forced Washington area schools to close for so many days this winter, school officials are trying to adjust schedules to make sure kids are in class for 180 days. That has left many kids and their parents wondering how and why 180 days became the magic number for school days. Valerie Strauss explains.