Conchita Mitchell, president of the Arlington County Council of PTAs, was appointed last night by the County Board's Democratic majority to a four-year term on the School Board.

She will succeed Margaret A. Bocek, the only Republican on the five-member School Board, whose term expires June 30.

Mitchell, whose father, Ernest Saulmon, served on the county's School Board in the late 1960s, said she is "pleased with the Arlington schools." She was selected from a field of six finalists for the $5,000-a-year job.

A veteran School Board observer who attends most of its meetings, Mitchell said she intended to join the other members in continuing the emphasis on capital improvements to school buildings and grounds and wants to examine staff development and salary improvements.

"It's time to look at the salary structures and make sure we're paying all school personnel , not just the teachers, a sufficient amount of money to make them want to stay in Arlington and remain committed to it," she said, noting predictions of a teacher shortage.

The County Board's four Democrats unanimously approved her appointment last night. Michael E. Brunner, the board's only Republican member, abstained to avoid a potential conflict of interest.

Virginia's Supreme Court, in a 1984 opinion, ruled that elected officials could not participate in school-related votes if they have a spouse who works for the school system. Brunner's wife Elizabeth is a part-time school psychologist.

The other County Board members have criticized the court ruling, which arose from a case in Richmond. Efforts in the last session of the Virginia General Assembly to get the law amended for persons in Brunner's situation were unsuccessful.

Mitchell was nominated by County Board member Ellen M. Bozman, who said the new School Board member would "serve the children of Arlington exceedingly well."

County Board member Albert C. Eisenberg said Mitchell has "a proven depth of knowledge" on school issues and "knows the schools from north to south [Arlington] . . . and is sensitive to their place in the current of community life."