Arlington School Superintendent Charles E. Nunley has questioned the legality and the fairness of a County Board decision last month to set aside $250,000 for extra increases in teacher salaries next year.

The special fund has also been criticized by the board's two Republican-backed independent members, both of whom voted against the measure when it was approved by three Democratic members on April 23.

"It is well known that many teachers work for and campaign for Democrats," said independent County Board member Walter L. Frankland Jr. yesterday. "This is, in effect, a payoff to teachers . It's a blatant, outright payoff."

Democratic board member John G. Milliken led support for the fund. "I'm disappointed there appears to be some reluctance about this," he said yesterday. "It's an opportunity for the school board to look at teacher compensation and make some necessary and important changes."

Milliken, who denied the funds were a "political payoff," cited national studies on how poorly teachers are paid, and local data showing most county teachers getting less pay than those in nearby localities.

In a memo last Friday to the School Board, Nunley said he had reservations about whether the County Board had the legal authority to tell the School Board that it should specifically spend the money on salary increases. Nunley also cited administrative difficulties in doling out the money to the county's 1,005 full-time teachers.

Nunley also said that "while I agree that teacher salaries, in general, are too low" he was concerned about the fairness of giving teachers, but not other school system employes, a pay increase beyond the amount already set out in the county and school budgets.

Nunley noted in his memo that the extra $250,000 for the teachers would amount to a 4 percent salary rise next year, compared to 3 percent for other school and county employes. The $250,000 would be added to the $51.3 million in county funds already allocated for the schools in fiscal year 1984, which begins on July 1.

The School Board is expected at its meeting tomorrow to review ways the money could be spent.

If the money is accepted for teacher salary increases, Nunley suggested various options, including a $200 across-the-board raise for teachers, an increase in the longevity stipends or special stipends to attract teachers in "critical subject areas" such as as math, science and industrial arts.

The county attorney's office said yesterday it had not been asked yet to review the legality of the County Board's action. School Board Chairman Evelyn Reid Syphax said that if the School Board accepts the money--as expected--it would be required to follow the county board's directive on how to spend it.