The public school year, as most of us know, is 180 days long. Except that usually it isn’t, and this year it really isn’t.

Consider the Concord school district. Its initial calendar for middle and high school, sent home to parents in September, had 177 instructional days – not including four professional-development days where students didn’t attend – between Aug. 30 and June 14.

(If that sounds short, consider that nine other districts also scheduled 177 days, while 17 districts set shorter school years, down to 169 days for Teams Charter School in Penacook. The other 157 districts monitored by the state Department of Education scheduled longer school years, mostly 180 days long with a couple as long as 183 days.)

Then came winter, which caused six snow days and five delayed openings in Concord. To make up for that, Superintendent Terri Forsten wrote in a letter sent to parents last month, that the district is adding two school days.

When all is said and done, this year’s tally for upper grades in Concord is 177 minus six plus two days – or 173 days.

That’s a full 3 percent less than the 180-day standard which was required before 2007. So does that mean students are getting 3 percent less school time this year than they did a decade ago?

Not necessarily. The key is “instructional hours.”

The traditional 180-day school year wasn’t always here, but developed over many decades as the public school system evolved. A century ago, for example, many New Hampshire school districts had an academic year of three separate 10-week periods, which, in the days of scattered one-room schoolhouses, might be taught by three different teachers.

Eventually, the school calendar was codified in New Hampshire with each day having a minimum of 5½ instructional hours for grades six through 12, or a total of 990 hours in the year. Younger grades had 5¼ instructional hours a day, or 945 hours annually.

Then, in 2007, this calendar was made optional for school districts as part of changes to the Minimum Standards for School Approval designed to “allow schools to develop innovative pathways to address students’ learning and personal needs” by creating more flexibility.

The 2007 change required public school districts to provide at least 990 or 945 instructional hours per year, but said they could divvy them up any way they wanted.

There are a few restrictions. A school day can have no more than 8 hours of instruction for older students or 6 hours for younger students (kindergarten is different), and cannot be less than 3 hours. Private schools have almost the same requirements, except they face no cap on maximum instruction hours per day. Home schooling has no requirements at all.

Many public school districts, including Concord, have switched to measuring instructional hours rather than school days. Most, also including Concord, have far more than 990 instructional hours in their year, which allows them to lose some days without running into problems.

Even with only 173 days, for example, the school year for Concord High will include 1,081.25 instructional hours, Forsten wrote. That’s 6½ hours per day, out of the roughly 7 hours between the morning and afternoon bells.

Getting more instructional hours in the school day is one of the arguments for switching high schools to block scheduling, as Concord has, because it reduces the number of times that students have to switch classes.

State law requires school districts to have at least five extra days ready at the end of the year to handle snow days, because changing school calendars on the fly can be a problem for other schedules, particularly high school graduations.

Some years, snow days or other closures can push a district’s school year so late that seniors can’t fit in the necessary number of instructional days before graduation day – a date that nobody wants to change because so many other plans are built around it.

Districts facing this problem have been known to schedule school on Saturdays, to extend every school day by a few minutes to build up more instructional hours, and to beg the state Department of Education for a waiver. The state Board of Education has been discussing this issue to see if there are other options.

As for Concord, school will last until Friday, June 16 with graduation being held the next day.

Unless we have another snow day, that is. Then, who knows what will happen?

(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek.)

David Brooks can be reached at dbrooks@cmonitor.com. Sign up for his Granite Geek weekly email newsletter at granitegeek.org.