Joan McDermott, an English teacher at Kilmer Intermediate School in Fairfax County, and Peggy Keating, administrator of the Child Development Center in Falls Church, have joined the list of candidates for the Arlington school board seat being vacated in July. The post being sought by six candidates is now held by Ann C. Broder, whose term ends July 1.

McDermott, who lives in Arlington where her three children attend school, has been active in Arlington school affairs for several years. She is chairman of an advisory council on instruction and is a member of the six-year school planning committee.

McDermott sahd she is concerned that Arlington schools "are spreading themselves too thin" with too many "alternative" education programs and doubt that the county can afford them.

Keating, an Arlington resident who for 10 years has been an administrator at the private school for neurologically impaired children, said she is concerned that the needs of special education and gifted and talented pupils are not being adequately addressed in the public schools.

Although Simone (Sim) J. Pace has been all but officially appointed to the board seat, McDermott said she plans to stay in the race because "it's important for citizens to make themselves available, to let the county know others are interested."

Keating is also distressed by the appointment system, but said she will remain a candidate. She will not, however, participate in a candidates forum to be held at 7:30 tonight in the Little Theater at Washington-Lee High School.

McDermott, Pace and the other three board candidates -- E. Eugene Hubbard, Dillard C. Laughlin and Karen Rosenbaum -- will participate in the forum.

The forum will be televised live in Channel 30 for cable TV subscribers.