The Alexandria School Board voted unanimously last night to ban smoking by students at the city's only high school, T.C. Williams.

Student smoking was already banned in elementary and intermediate schools. Teachers and other adults will continue to be permitted to smoke in designated school areas.

Previously, students at T.C. Williams were permitted to smoke in designated areas.

The vote ended a confrontation between the board's four smoking members and its five nonsmokers.

Board member Judith Seltz, a nonsmoker, proposed a tougher version of the policy last month, to ban smoking by both students and adults. With one nonsmoker absent from that meeting, the policy was defeated on a 4-to-4 tie, with the smokers voting against and the nonsmokers voting for.

Last night Seltz said the watered-down smoking ban "is better than nothing; it's a start."

In other action, School Superintendent Robert S. Peebles, who was scheduled to retire June 30, agreed to hold the post through July 31 so the board will not have to appoint an interim superintendent.

Board Vice Chairwoman Sandra Lindsay announced that the board has narrowed its search for a new superintendent to seven candidates, who will be interviewed next month. The names of the seven have not been made public. The board hopes to select a new superintendent by August.

The board voted 6 to 3 to appoint Robert F. Yeager as principal of George Washington Junior High School. He succeeds Steve Osisek, who is retiring after 35 years with the school system. Yeager has been associate principal at T.C. Williams High for three years and was associate principal of Francis C. Hammond Junior High from 1979 to 1984.