Fairfax County School Superintendent Robert R. Spillane last night withdrew his plan to extend Mondays to a 6 1/2-hour day for elementary students, an idea that has drawn loud opposition, replacing it with a proposal to make each school day six hours long.

While the Monday schedule would be lengthened from its present four hours for elementary pupils, the other four days of the week would be shortened by 30 minutes each, creating a uniform week with virtually no change in the overall time spent in class.

Spillane portrayed this plan, which he said would not cause additional expenditure, as an interim step toward eventually keeping the county's 72,000 elementary students in school 6 1/2 hours a day all week, as he originally wanted.

"The 6 1/2-hour day doesn't seem able to fly," he said of his original plan, which would add $3.6 million to school spending.

The move is the latest twist for a plan that has seemed dead and been resurrected several times in the last year and a half.

The current elementary school schedule, in place for nearly 20 years, enables teachers to spend Monday afternoons planning and conferring with parents.

Spillane and School Board Chairman Kohann H. Whitney (Centreville) have pushed for an end to that practice to increase instructional time for students. For more than a year, both repeatedly ruled out the idea of a uniform six-hour day because it would not affect the net time in class. Spillane conceded last night that the latest version would do "absolutely nothing" for instruction.

But a torrent of criticism from union leaders and PTA activists at public hearings Monday and Tuesday nights persuaded Spillane to back down, and most board members said last night they were willing to consider the new plan.

"That's a beginning, I guess, a step in the right direction," Whitney said.

"Better to get half a loaf of bread than no bread at all," added member Anthony Cardinale (Springfield).

In numerous public meetings in recent months, the six-hour day plan has drawn less hostility from teachers and parents but also generated less enthusiasm among those favoring a longer Monday.

Some teachers at schools that tested a uniform six-hour day several years ago have called it preferable because it still allows planning time.

Maureen Daniels, president of the 6,900-member Fairfax Education Association, said she opposes the new proposal because there are too many unanswered questions and too few possible benefits.

"What would be the point?" she asked. The only purpose, she said, was to make day care easier for parents, and while that is a concern, "that's not a legitimate enough reason."

In contrast, Mark Glofka, vice president of the 1,500-member Fairfax County Federation of Teachers, said he favored the plan as long as planning time is guaranteed before or after school.

Spillane's switch represented a victory for teachers, Glofka added.

"He lost . . . . He tried to buffalo his plan to make change for change's sake and he got caught."