It's summer vacation for the kids. Why?

Fiscal limitations, century-old developmental theories, and outdated medical concerns: The now-standard 180-day academic calendar with a long summer holiday didn't come about until the early 20th century. Previously, urban schools operated year-round with short breaks between quarters.

In 1842, Detroit's academic year lasted approximately 260 days, New York's 245, and Chicago's 240. But since education wasn't mandatory in most states until the 1870s, attendance was low.

Despite the official schedule, many kids ended up spending the same amount of time in school back then as they do now. Brooklyn school officials, for example, reported in 1850 that more than half their students showed up just six months a year.

Poor attendance got some people wondering if such a long academic calendar was worthwhile. Why keep schools open year-round if most kids don't even go? Reformers also warned that goody-goodies who did show up every day might burn out. Many physicians at the time felt that students were too frail, both in mind and body, for so many days at their desk. Too much education, they argued, could impair a child's health...

...Physicians no longer believe that children are too feeble for year-round instruction, and most school buildings now have effective ventilation systems. So why don't we go back to having school in the summertime? For one thing, it's expensive to keep schools open, just like it was in the late 1800s. But some nonprofit organizations argue that the long breaks hinder the learning process.

According to the Johns Hopkins Center for Summer Learning, kids score worse on standardized tests in early September than in late June. Plus, students in other industrialized countries have more instructional time. The Israeli academic year lasts 216 days, and kids in Japan plug away for a whopping 243 days per annum.

This from Slate.