At the first joint budget meeting between the Loudoun County Board of Supervisors and the county School Board on Tuesday, the stage was set for tense negotiations over the school system’s adopted $949 million budget, a funding request that has drawn support from educators and parents but skepticism from county leaders.

School Board Chairman Eric Hornberger (Ashburn) said the school system’s spending plan, which would require about $638 million in county funds, is necessary to keep pace with three new schools and the addition of more than 2,300 students during the 2014-15 academic year.

Hornberger pointed out that the fiscal 2015 budget marks a 27.4 percent increase in funding over the school system’s fiscal 2009 budget, but that within that six-year period, 12 new school facilities opened and the county’s public schools absorbed more than 16,000 additional students.

Despite the burgeoning student population, Hornberger said, “deferments had to be made in a number of areas” as a result of reduced funding.

But the system can no longer afford to follow that pattern, he said, because improved technology, smaller class sizes and more competitive teacher salaries are urgently necessary costs.

“Further deferments of these expenditures are no longer prudent, as they are already beginning to be felt in the classroom,” Hornberger said. “We are a growing school system serving a community with high expectations. We can no longer afford to further postpone or avoid necessary expenditures to move our school system forward.”

Supervisors had asked County Administrator Tim Hemstreet to prepare a county budget assuming an equalized real property tax rate of $1.155 per $100 of assessed value, which would avoid increasing taxes for average Loudoun homeowners.

At an equalized tax rate, the school system would face an anticipated funding gap of roughly $40 million. The school system could also face additional financial challenges as a result of reduced state revenue, supervisors Chairman Scott K. York (R-At Large) said Tuesday. The current projection for cost-of-competing state education funding — money to help Northern Virginia schools attract educators in a highly competitive market — is just $5 million for the region; the county had originally hoped for $7 million for Loudoun alone, York said.

“We’re in one of those situations where, again, the state is not really holding up to the agreements that it has made,” York said.

Several supervisors questioned whether full funding was necessary for the school system to meet its top priorities. Supervisor Ralph Buona (R-Ashburn) quickly pointed out that the county has continually raised its level of funding for the school system in recent years and that key goals. such as raising teacher salaries and reducing classroom sizes, could still be fulfilled at an equalized tax rate.

Buona also sought to justify a recent memo sent to the School Board from the Board of Supervisors that asked School Board members for more detailed information about their funding priorities. The information was necessary, Buona said, for the board to better understand the implications as its members set a property tax rate.

The memo “was not done to usurp your authority; it was for us to help understand priorities, because otherwise we operate under this all-or-nothing, take-it-or-leave-it kind of proposal,” Buona said. “Tell us what more money does for you.”

Toward the end of the joint meeting, School Board member Tom Reed (At Large) said that it would have been better to establish that understanding “at the beginning of the process, not when we’re 90 percent through it. If we had been involved in the conversation early on and you understood what we felt were the priorities, I think the conversation would have been a lot better and more informed.”

Board of Supervisors Vice Chairman Shawn Williams (R-Broad Run) appealed to the school system leaders to consider what he said was the permanent solution to budget woes: luring businesses to Loudoun to strengthen the county’s commercial tax base.

“If we increase taxes too much . . . we lose corporate investment in our jurisdiction to neighboring jurisdictions, and we won’t get that commercial tax base,” he said. “Please don’t impair our ability to find a real solution to growing revenue in our county.”

York echoed that sentiment at the end of the meeting, calling the budget process “a very difficult balancing act.”

Compromise on both sides would be necessary, York bluntly told School Board members.

“At the end of the day, we’ll come out with something that nobody likes but everybody can live with,” he said. “Because I can tell you, this board isn’t going to do full funding.”