The chairman of the Fairfax County Board of Supervisors has accused the School Board of conducting illegal negotiations with teachers' representatives in reaching agreement to seek 10.4 percent teacher pay raises.

Chairman John F. Herrity has summoned school board members to a Monday morning meeting with the supervisors at which he said he will castigate them for the manner in which they "negotiated contracts with a labor union, the FEA (Fairfax Education Association)."

A 1977 ruling by the Virginia State Supreme Court denies public employes the right to collective bargaining.

Fairfax school officials say that since mid-September school administrators and FEA members met privately to work out an agreement on salaries and working conditions. They say these "meet and confer" session were conducted under guidelines approved by Attorney General J. Marshall Coleman.

Herrity sees those session in a different light. "It seems to me," he said, "when the school staff and the school board negotiate with a labor union -- the FEA -- and no public acknowledgement is even made that the negotiations are going on, we've got plenty to talk about."

The meeting resulted in agreement on a proposal to raise teacher salaries by a total of $22.7 million, accounting for all but $4.7 million, accounting for all but $4.7 million of the spending increase called for in the school superintendent's budget recommendations. County officials estimate that the total $27.4 million increase would require raising property taxes on the average county home by $113.

Board of Supervisors members say that since the school budget proposal was released last week they have been swamped with calls from angry citizens asking them not to fund a budget that could result in higher taxes.

School board members began preparing at their Thursday night meeting for their Monday command appearance before the supervisors. School Board Chairman Rodney F. Page did not adjourn the meeting as scheduled, but, following a brief recess, had stacks of budget documents distributed to board members. The documents were described as copies of reports being sent to the supervisors.

The school board members looked the documents over and limited their discussion to several points concerning Gov. John N. Dalton's proposed state budget.

One school official, who asked not to be identified, said later Page "obviously was hoping the press and public would be gone and there would only be board members and school staff present. Because (the press) was there, several things weren't discussed which I'm sure would have been."

"Well," said Page, "I don't think we expected everyone to leave, but we didn't expect everyone to stay, either. I thought the board members would want to go through it (the reports) thoroughly and it might take a great deal of time. Draw your own conclusions."

During the meeting, school staff members said the budget controversy arose because the school budget is released earlier than the county government budget proposal and is held up to public scrutiny for a longer period.

"At this time we are out in front of the county board by two months," said Myron Cale, director of school financial services."If the two executives issued their advertised budgets at the same time, we'd be home."

The FEA claims membership of 6,500 of the county's 7,200 teachers. Its president-elect, Bill Costello, expressed surprise yesterday at Herrity's accusation. There was nothing illegal about the discussions between the administrators and the association, he asserted.

At least one school board member had second thoughts about the discussions. "In retrospect, I don't agree we should have done it that way," said board member Gary Jones. "When 85 percent of your budget goes to personnel costs, your staff should not be reaching a private agreement with the teachers."