Teacher union leaders and PTA activists urged the Fairfax County School Board last night to dump the plan by Superintendent Robert R. Spillane to extend the school day on Mondays for elementary students. Some urged dumping Spillane as well.

In round three of the battle over the Monday plan, most of the largest community groups in Fairfax castigated Spillane's attempt to revive a proposal that was rejected by the board as recently as 10 weeks ago.

For more than 1 1/2 years, Spillane and School Board Chairman Kohann H. Whitney (Centreville) have championed the idea of abandoning the longstanding tradition of closing elementary schools up to 2 1/2 hours early on Mondays. Although the board approved the concept in November 1989, a $5.9 million plan to implement it failed on a 5 to 5 vote in November 1990, largely because of the bleak economic picture.

Spillane, arguing that the extra time in class for the 72,000 elementary school students is crucial, pared down the cost to $3.6 million and resubmitted it this month as part of his 1991-92 budget proposal.

At a public hearing last night, teachers said the latest plan is worse than the earlier version because it would hire lower-paid instructional aides instead of teachers to make up for the planning time homeroom teachers would lose on Monday afternoons.

Parents, including some who supported the earlier proposal, generally called this one unsound and unaffordable at a time when teachers will receive no raises and other spending is being frozen.

"If we couldn't afford it then, how can we afford it now?" asked Stephanie Long, PTA president at Cub Run Elementary School. "If this proposal is so educationally sound, why wasn't it presented initially?"

Probably the most caustic personal attack on Spillane came from Maureen Daniels, the president of the 6,900-member Fairfax Education Association.

"We are not nearly as distressed by current economic difficulties as we are by Dr. Spillane's cavalier and callous actions," she said. "We know the economy will improve, but it is now clear to us that Bud Spillane will not.

"This latest scheme has only its cheapness to recommend it," she added. "It has little to do with improving instruction and everything to do with ego and power and an obsession with winning -- at all cost. It is the Spillane style."

Several other speakers suggested that the board find a replacement for Spillane.

"You need to hire a superintendent who will afford teachers the respect and dignity which they have earned for the work they do in the classroom," said Rick Nelson, president of the 1,500-member Fairfax County Federation of Teachers.

A few speakers held out for the Monday plan, including the Fairfax County Council of the Arts and the county Federation of Citizens Associations. Increasing class time "offers a welcome chance to expand the time devoted to the core curriculum," said parent Ginger Shea.

The board, which is deeply divided over both the Monday plan and Spillane, will hold another public hearing tonight and then vote on Feb. 19.