Arlington School Superintendent Larry Cuban last night proposed giving teachers and other employes 10.5 percent raises as an "unambiguous signal" that their welfare is the school system's concern.

But the $53 million school budget that Cuban gave the county school board last night did not satisfy either the teachers' organization or some members of the County Board, which provides most of the money to run the schools.

The proposed raise is "still nearly 3 percent under the rate of inflation," said Marjorie Sale McCreery, president of the Arlington Education Association, which represents 90 percent of the county's 1,000 teachers. McCreery said the raise would not be enough to halt the "work-to-the-rule" job action that Arlington teachers began a year and a half ago in a salary dispute.

"I don't want to appear to be an ingrate, but nevertheless, it's a cut in pay again," said McCreery.

Members of the Republican-backed County Board majority said they were unhappy with the amount the county government would have to provide.

Earlier this year, the County Board said the school budget should require $38.3 million that the county is providing for school operations during the current fiscal year. Cuban's budget proposal exceeds that guideline by 3 percent, or $1.1 million.

"I think our guidelines for the school budget were very realistic," said County Board member Dorothy Grotos. "I'm a bit disappointed."

In his presentation to the school board, Cuban referred to the 5 percent guideline as a "floor." He said the additional 3 percent he is seeking represents a "modest ceiling . . . I commend the County Board for giving the school board a flexible guidline."

"Any kind of guidelines we gave him were ceilings," said County Board Chairman Walter Frankland, who is a vocal critic of Cuban and the school board.

Cuban told the school board, which still must approve the proposed spending program, that 90 percent of the increase in this year's budget was made up by a $4.1 million package of salary and other employe compensations.

But Frankland accused Cuban of presenting the budget in that light as a ploy. "Invariably the school board comes in higher than the guidelines. Then if any cutting is done, the County Board takes the heat. That's their strategy."

Salaries and related items comprise 85 percent of Cuban's proposed budget. Other budget items include a $76,000 increase in gasoline costs, $73,000 in heating fuel and $360,000 for textbooks, instructional materials and library books.

The proposed budget estimates per pupil costs at $3,582 in 1980-81, a rise of $500 over the current school year.

Public school enrollment now at 15,761, is expected to drop by about 6 percent in the county next year, to 14,816.