Ending a 40-year practice of half-day Mondays, the Fairfax County School Board voted Thursday to fund an effort to extend Monday classes for elementary schools, adding an extra 75 hours of instruction for students in kindergarten through sixth grade.

The board voted unanimously to spend $7.6 million to fund the plan, which had been approved in late June. The money came from $38 million in leftover cash from the prior year’s budget.

School Board members had originally planned to pay for the effort with money from the Fairfax Board of County Supervisors. But at a July 1 meeting, some supervisors cast doubt on that prospect. It is still possible that the supervisors could approve some surplus money for the schools project in September.

“I do know that the Board of Supervisors carried over the item for funding for full-day Mondays for consideration as they look to their final budget in early September,” School Board Chairman Tammy Derenak Kaufax (Lee) said at the meeting. “I do urge them to support our efforts by sharing the costs of this funding of this calendar change as they make their decision in September.”

In total, the School Board voted to spend $15.1 million of the surplus money. In addition to the $7.6 million to extend Mondays in elementary schools, those expenditures include funds for a synthetic turf field initiative, new buses and building maintenance projects.

“It is extremely thoughtful year-end spending,” said board member Megan McLaughlin (Braddock).

School Superintendent Karen Garza said that the administration’s $38 million in surplus cash was 1.5 percent of the school system’s overall $2.5 billion budget. She also noted that this year’s surplus was smaller than last year’s $55 million in leftover money.

Board member Elizabeth Schultz (Springfield) noted that the board could have voted to give employees a slight salary increase with the extra money by moving forward by one or two months a step-increase scheduled for November. Such a move would have cost $4 million or $8 million, respectively.

Chief Operating Officer Susan Quinn said that doing so would leave the schools with a smaller amount of money to carry forward to the next fiscal year. Board member Jane K. Strauss (Dranesville) said that it is illegal under Virginia law for the schools to end the fiscal year in debt.

“We can’t raise money, and we can’t run in the red,” said member Pat Hynes (Hunter Mill). She said that the supervisors have put the administration at a disadvantage next year by leaving the schools “tens of millions of dollars short.”

Hynes also said that although she is seeking better compensation for schools staff, “a couple of months of step is not going to make a big difference.”

According to the 2014 Washington Area Boards of Education guide, Fairfax County’s average salary for mid-career teachers ranks behind school districts in the cities of Alexandria and Falls Church and in Arlington, Montgomery, Prince George’s and Prince William counties.