The Arlington County School Board has decided to freeze all hiring and purchasing in response to an unexpected shortfall of $888,593 that developed this summer.

The shortfall results from a decision by 209 Arlington teachers to take their salaries over a 10 - month rather than a 12 - month period. To comply with this decision, the school board must find in the current year's budget two months worth of teacher salaries which otherwise would have been paid out of the fiscal '79 budget.

The freeze, which the school board approved unanimously last week, leaves at least six teaching positions vacant, and prevents any new teachers from being hired for positions that become vacant during the school year. Exceptions may be made on approval by the school superintendent.

In additon, approximately 12 other "support" positions already budgeted, including custodial, secretarial and maintence positions, probably will not be filled as a result of the freeze.

The freeze, however, does not jeopardize the positions of 55 teachers who were recently hired by the school board but who had not signed contract.

"I don't know how we are going to solve this deficit," said school Supt. Larry Cuban. "There's no easy way to prevent us from going into an emergency situation again as we have had to do in the past, but we are going to try to avoid that."

School spokesmen said that the school system last year saved only $100,000 by imposing a hiring freeze in response to a tight budget. The school board plans to confront the county board with the deficit problem at ajoint meeting Monday.

"Hopefully we will be able to come up with some combination of budget reductions and county help to solve this problems," said school board chairman Thomas Penn.

The deficit appeared in mid - August, when 223 Arlington teachers, in signing their contracts for this school year, opted to receive their paychecks during the months. Teachers have had the option of receiving their salaries over a 10 - month period since 1955. However, until this year far fewer have taken it. Fourteen teachers have since changed back to a 12-month pay plan at the request of Supt. Cuban.

Last year, only 170 teachers chose to be paid within 10 months, school spokesmen say. Now, a total of 379 teachers have requested all their paychecks by June, which means they must be financed from the 1977 - 78 budget.

The Arlington Education Association (AEA), a teachers organization that represents nearly the entire teaching staff of Arlington schools, encouraged teachers in early June to request 21 pay periods rather than 26.

The AEA recommended that teachers be paid within the school year and put extra money from each paycheck in a local credit union or savings account to gain interest that would soften the effects of the 2 per cent pay increase granted this year by the Arlington county board. The AEA termed the increase "paltry."

"This was not a concerted job action," said AEA president Lee Bosper. "But the AEA has a responsibility to look out for the economic situation of the teachers. We pointed out to them that they might as well gain a little interest by having their salaries in interest - bearing accounts."

In a school opening speech Cuban had requested teachers who opted for 21 pay periods to reconsider and take 26 pay periods, saying that teachers "probably did not realize what the collective impact would be of their individual decisions." The deadline for any more teachers to change their minds is Wednesday.

AEA spokesmen say the deficit is a temporary one, that could be covered either by county board for the last two months prior to the 1978-79 fiscal year, or by the school board taking out a loan.

Cuban says neither of the proposed solutions are suitable for relieving the deficit.

The school board plans to withhold the option for teachers to choose 21 or 26 pay periods from the 55 new teachers who have not signed contracts. They will be paid over a 12 - month period.