Prince George's County school Superintendent Edward J. Feeney yesterday proposed a $28.2 million increase in next year's school budget, a proposal that was immediately challenged as "virtually unfundable" by the county's budget director.

The 9.1 percent increase would bring the school budget to $336.5 million, with about 63 percent of that coming from county funds.

"We have reached the point where we have trimmed as far as we can trim," Feeney told school board members last week. And yesterday, school spokesman Brian J. Porter said the budget squeeze is "fairly serious." "The level of funding," he said, "can only be called adequate. If inflation is worse than we presume," the budget request "might not be enough."

But Robert O. Duncan Jr., director of the county's budget office, said it appeared "technically impossible" to meet the request. Even if no other county-funded services required budget increases next year, he said, the proposed budget would force the county to exceed the $143.5 million limit on property taxes imposed by the 1979 TRIM charter amendment.

County property tax revenues are currently about $9 million short of that limit, he said. It is legally impossible for the county to modify TRIM, to allow for more tax revenue, before the November 1982 elections. Duncan said there is "no other significant tax source" from which to draw funds for the school budget.

He said this year's school budget depended on a county fund surplus remaining from the previous year. That surplus, estimated at about $17.7 million, has since fallen by approximately $10 million, Duncan said. The school system's own surplus is $4.3 million; last year it was $11.9 million.

A major proposed budget increase involves replacing 111 twelve-year-old buses -- for $2.5 million -- as required by state law.

The cost of salaries would go up $10.1 million, because of wage increases, despite elimination of 328 staff positions. Driver education, currently provided free of charge, would become an after-school program for which students would be charged.

The proposed budget also includes the closing of eight county schools -- already scheduled as a result of a 1977 decision. At the same time, a new science and technology center is due to open in September at the Oxon Hill Senior High School.

Total enrollment is expected to drop by 5,100 students, to 111,100.

Porter said that consumer prices increased 49.6 percent over the last four years while the school budget has increased only 27.5 percent. This year, he said, Superintendent Feeney has "laid it right on the line -- the people will have to own up to a responsibility to public education in this county."

School board member Jo Ann Bell, who has just just stepped down as chairman, said the school system cannot cut back on its budget any further without seriously damaging standards. If the county cannot provide the money because of TRIM, she said, then "obviously something's got to give."

Three public hearings have been scheduled for the beginning of the year on the budget. It then goes to the school board, is passed on to the county executive, and goes finally to the County Council.