Fairfax County residents oppose a longer elementary school day but generally support other changes recently implemented or being considered for their schools, according to a survey released last night.

In a stinging rejection of one of the county's top educational thrusts this past year, 49 percent of the respondents in a poll of county parents and others disapproved of longer hours for grade-school children, while 43 percent approved.

"I don't think it's been sold appropriately," consultant Marvin J. Cetron told the county School Board. "I don't think the parents understand why we need more time."

The board has approved in concept a plan to eliminate early Monday closings of elementary schools beginning in September 1991.

Currently, students get out 2 1/2 hours early on Mondays, allowing teachers a block of time for planning; under the new plan, students will be in school 6 1/2 hours each weekday.

While the school-commissioned survey didn't ask specifically about that plan, observers said the response showed ambivalence over any change.

"They clearly need to reexamine and reopen the issue," said Maureen Daniels, president-elect of the Fairfax Education Association.

"There's no mandate. We have to ask ourselves: What are we restructuring for?" he said.

School Board Chairman Kohann H. Whitney has pushed the plan in part to make child care easier on Mondays and in part to allow more teaching time.

However, teachers have fought the idea because they don't want their Monday planning time spread over five days.

Jane Strauss, president of the County Council of PTAs, said parents are not necessarily opposed to any change, they just don't want it at the expense of teacher planning time and unpopular shifts in bus schedules.

"There isn't a huge groundswell of {people} saying, 'We will put up with many inconveniences and no planning time for the sake of implementing this plan,' " Strauss said. "In other words, all those factors have to be considered."

A task force is studying how to implement the new schedule, and the School Board is expected to vote on its funding and other details this fall and next spring.

The survey did not ask about the move to add 30 minutes and a seventh period to the intermediate and high school schedule this fall, a plan that also has met with strong opposition from teachers and parents.

In other areas, the survey showed strong community support for the school system's direction.

About 84 percent of the 1,033 parents and other adults interviewed from March 28 to April 9 rated the county's public schools good or excellent, and majorities supported teacher merit pay, greater school-based decision-making, full-day kindergarten and preschool programs for 4-year-olds.

Despite concern about minority achievement, blacks and Hispanics were more likely than whites to say the schools are "getting better," according to the poll.

Other findings included:

Drug and alcohol abuse and student discipline were seen as the two biggest problems facing the schools today. More than half of those surveyed agreed that discipline is "not strict enough"; the percentage who thought this was even larger among blacks, Asians and Hispanics.

Priorities for more funding should include substance-abuse programs, dropout prevention, counseling services and programs for students with disabilities, gifted students and minorities. However, only 24 percent of the respondents said they would be willing to pay higher taxes for more school spending.

Teachers have the highest approval ratings of school system employees, with 72 percent of the respondents citing them as good or excellent, compared with 62 percent for principals, 46 percent for Superintendent Robert R. Spillane and 42 percent for the 10-member School Board.

The community believes the schools need to do a better job preparing students for the work force; 70 percent said vocational training should be expanded.

.........FAIRFAX SCHOOLS' COMMUNITY SURVEY ON EDUCATION ISSUES....

Fairfax Schools'Biggest Problems..................................

Student abuse of drugs/alcohol................40%.................

Student attitudes/behavior/discipline.........23..................

Poor administrators and teachers..............12..................

Lack of parental involvement..................11..................

Fiscal concerns...............................11..................

Lack of emphasis on basic skills..............10..................

Quality of Personnel....Teachers....Principals....Supt..School Bd.

Excellent.................26%..........18%.........7%......5%....

Good......................46...........44.........39......37.....

Fair......................11...........13.........21......25.....

Poor.......................2............3..........5.......5.....

Don't Know................15...........22.........27......27.....

Reform Initiatives............Approve................Disapprove..

Teacher merit pay................71%....................24%......

School-based management..........77.....................12.......

Longer elementary school day.....43.....................49.......

Longer school year...............46.....................47.......

Full-day kindergarten............59.....................35.......

Preschool for 4-year-olds........70.....................25....... SOURCE: Survey by Marketing Opinion Research Inc. and Forecasting International Ltd.