With classes canceled across the Washington region for Tuesday’s snowstorm, some school districts are preparing to add days to the academic calendar to make up for this school year’s winter closings.

In Virginia, Maryland and the District, students are required to attend school for 180 days each year, and school districts must add days to the academic calendar in the event that inclement weather cancels classes. Although most school systems build in buffers, schools in Fairfax County and the District, for example, already are adding days to the school calendar one month into winter.

In Fairfax, Tuesday’s cancellation was the fourth this school year. After forecasters called for snow showers to continue into the late evening Tuesday, administration officials decided to cancel classes Wednesday. Fairfax administrators already have decided that the school system will have classes Feb. 17, the Presidents’ Day holiday, to make up for Tuesday’s snow day. An additional makeup day is set for April 7, a teacher workday.

Jeff Platenberg, assistant superintendent for facilities and transportation in Fairfax, acknowledged that it can be a hassle for some parents to take leave from work to stay home or make last-minute plans for a babysitter when school is canceled. But student safety is always the priority, he said.

“An inconvenience is strictly that — it’s an inconvenience,” Platenberg said. “Every decision, I’m thinking: What’s in the best interest of our students?”

Melissa Salmanowitz, a spokeswoman for D.C. public schools, said that the city’s school year will be extended in June for the two school days lost to snow. D.C. schools did not close this month when a cold snap brought near-zero temperatures to the region.

Virginia requires school districts to offer a minimum of 990 instructional hours each year, which is the equivalent of 180 teaching days with 51 / 2 hours of academic instruction daily. Phil Kavits, a spokesman for Prince William County schools, said the district began the school year with about eight extra days built into the schedule because of a slightly longer school day — 5 hours 45 minutes — leaving the district with a few days to spare.

“But we have to keep a close eye on this at the rate we are going,” Kavits said.

Loudoun County, with a minimum of six hours per school day, builds in an even bigger cushion, with 15 extra days, said schools spokesman Wayde Byard.

“Even during Snowmageddon, we did not have to make up days,” Byard said, referring to the blizzard that shut down the Washington area in 2010. Loudoun also decided to close schools Wednesday because of the accumulation.

Alexandria has just three extra school days factored into its calendar. Tuesday’s school closure was the fourth this school year, said spokeswoman Kelly Alexander. She said the School Board will have to approve a plan for making up the lost time.

Prince George’s County schools spokesman Max Pugh said that the scheduled last day of school, June 5, might be moved back to make up for three days of canceled classes. But those extra days might not end up being used, Pugh said, as the state can grant “waivers” and forgive the lost time.

Schools in Arlington and Montgomery counties were scheduled to be closed Tuesday for teacher workdays.

Dana Tofig, a spokesman for Montgomery schools, said there is room in the calendar for one more weather-related shutdown before extra academic time is needed.

After three snow days earlier in the season, most of Arlington’s schools are still on schedule. But some elementary schools that have early-release days have begun adding hours to make up for lost time.