The lame-duck Montgomery County school board, in what was probably its last significant action as a conservative bloc, voted yesterday to ban smoking in county schools amidst speculation that the action would be reversed by newly elected school board members when they take office Dec. 1.

If allowed to go in effect next February, as scheduled, yesterday's vote would reverse a 15-year-old school policy that permits high school students who have written permission from their parents to smoke in specified areas. Superintendent Edward A. Andrews, who supported the change, estimated that enforcement of the new policy, which would require employing building monitors, would cost $330,000 a year.

The proposal was opposed by the slate of four candidates who swept the recent election, toppling the six-member conservative majority. The winning candidates said at a number of pre-election forums that they might reconsider such a ban if it was approved.

The new restriction would join Montgomery with the District as the only area school systems to ban smoking on all school campuses. Arlington, Alexandria and Prince George's schools allow smoking in designated areas.

In Fairfax County, individual schools determine their own smoking rules, and the majority permit student smoking in designated "smoking courts." But a growing number of Fairfax schools have decided to prohibit student smoking, according to school spokesman George Hamel.

Reston's South Lakes opened as the first "no-smoking" Fairfax school. It has since been joined by Lee, Jefferson, Herndon, McLean and Lake Braddock schools. For high schools that permit smoking, the school board supplies what are referred to as "health awareness aids" that are supposed to alert students to health hazards posed by cigarettes.

In yesterday's vote, Montgomery school board members Carol Wallace and Joseph Barse, who were defeated last week, along with Marian Greenblatt, Suzanne Peyser and Richard Claypoole, voted for the measure. They cited health reasons as their main objective. Blair Ewing and Eleanor Zappone opposed the motion.

"I vote for this with full knowledge and all likelihood that this action will be reversed by the new board come January 1," said Wallace, in response to a request by Ewing that the vote be delayed until the new board takes office. "But I will not abrogate my responsibility as a member of this board and I will not have it on my conscience that I might have taken some action that was to the future detriment of Montgomery County public school students."

James Cronin, the only newly elected board member present during the vote, did not indicate during the discussion whether he would move to reverse the decision, but after the meeting he said he would not vote to approve the cost of implementing it. Robert Shoenberg, another member-elect who was present earlier in the meeting, said later he "would have preferred if they left the policy alone."

It was the second time the board considered the smoking ban, which was opposed by a broad coalition of teacher, parent and student groups. The motion failed on a 3-to-3 vote last month when Greenblatt did not appear at the first board meeting following her defeat in the Republican congressional primary.

Barse, who earlier opposed the motion because of its cost, said he had changed his mind when it became evident through his sources "in the student underground and antidrug groups" that students were smoking marijuana in the designated smoking areas.

"That tipped me in the other direction," Barse said.

Student board member Kurt Hirsch, who does not have a vote, criticized the proposal as "virtually impossible to enforce." The prohibition, he said, would be just another "challenge" for students to beat.