Fairfax County elementary school students will receive an extra 75 hours of instruction next year after the school board voted late Thursday to eliminate half-days on Mondays.

The school board voted 10 to 1, with one board member absent, to end the 40-year practice in elementary schools.

The move will add 21 /2 hours of classroom time each week, which school officials said could help the administration address achievement gaps and improve performance for all students.

School board member Elizabeth Schultz (Springfield) noted that the new schedule will add more than 100 days of instructional time to each student’s elementary academic career.

Schools superintendent Karen Garza said that the change was inevitable.

“It needs to happen,” Garza said. “It’s quite obvious, actually.”

Getting rid of half-day Mondays in elementary schools was part of a plan to switch the school system’s calendar from 180 days of instruction each school year to 990 hours of instruction, which gives the school system flexibility for snow days.

The change will add 13 extra snow days into the calendar and probably will prevent the school system from having to tack on extra class time in June after particularly harsh winters such as the most recent one.

Garza acknowledged that the process of approving the calendar change was accelerated. Administrators presented the school board with the plan less than a month ago, and some teachers questioned whether the proposal was being rushed into place without further examination.

Fairfax Education Association Vice President Kevin Hickerson told board members Thursday that teachers said that their concerns about the proposal were not addressed.

Former school board member Stu Gibson told his former colleagues in testimony that he realized there was “nothing I can do to stop this headlong rush that your teachers tell you is not ready for prime time.”

Gibson also noted that the estimated cost of implementing the plan in the fall, at between $4 million and $7.6 million, could come at the expense of teacher pay raises and increased class sizes during budget negotiations next year.

“We know the timeline is aggressive, but it’s still the right thing to do,” Garza said. “It’s good for children.”

In a survey conducted by the school system, more than 78 percent of parents who responded approved of the proposal.

But 48 percent of elementary teachers who took the survey said they did not support the plan.