There is probably no more perilous column to undertake than one that has to do with school schedules. Having ventured onto that dangerous terrain in the past, I have some idea of the kind of heat that school administrators have to take when they do something as seemingly benign as shifting a half-hour here or there.

The roof caves in.

The teachers get up in arms if they see a threat to their paychecks or their planning periods, and the parents get upset if they see their children's schedules altered, and the school bus transportation people are there to complain that this will upset the entire transportation system. There are so many vested interests connected to the school days that it has become almost impossible to alter them without upsetting one or more powerful constituencies, and few school superintendents want to do that.

So it is important, in the debate raging over the extension of Monday school days in Fairfax, to acknowledge first-off that Superintendent Robert R. Spillane is showing considerable professional boldness in suggesting that the early closings on Mondays in the county's elementary schools are archaic and counterproductive.

The abbreviated Mondays were developed about 20 years ago to give teachers a solid block of planning time. Trouble is, the early closings also have given elementary school children a solid block of getting-into-trouble time. Instead of getting home after 3:30 or 4 p.m., depending on how long it takes the bus to tranport them, the youngsters are getting home around 1 p.m., often to unsupervised houses, because more than 66 percent of the mothers in Fairfax work outside their homes.

The school-age child-care programs that exist in many elementary schools take up the slack on Mondays for some children. But a lot of older elementary children refuse to go to the programs, which they consider to be for little kids. A lot of parents have developed work schedules that allow them to get home between 4 and 5 p.m. so that their children are not alone for long periods on Tuesday through Friday. Unfortunately, the children who might be alone for a half-hour on most weekdays may be left alone for three or four hours on Mondays. Monday is the orphan day in many parents' complicated child-care arrangements.

Spillane wants to have each elementary school day be 6 1/2 hours long beginning next September. This year, he added a half-hour to the county's middle and secondary school schedules, again after considerable controversy. Should he succeed in getting a uniform school day in the elementary schools, Fairfax youngsters would have the same amount of time in school each day as those in Howard and Loudoun counties.

The other counties in the Washington area average a six-hour elementary school day, which means that those youngsters are getting 4 1/2 hours less school time each week than those in Howard and Loudoun, or Fairfax, under the proposed reforms. Those additional half-hours, taken over the length of a school year, can add several weeks to the total educational time a youngster accumulates.

U.S. youngsters spend far less time in school than their counterparts in other industrialized nations. The Japanese school year, for example, is 243 days. The U.S. school year averages 180 days. U.S. youngsters are spending two full months less time in school than Japanese children. Meanwhile, industry is spending billions of dollars to teach literacy and other basic skills to a generation of workers that has entered the work force woefully ill prepared. Every month, it seems, a study comes out telling us that youngsters know nothing about geography and history. Assuming that children will learn more with more time in school, it seems only logical that U.S. schools should lengthen their days and their calendar year to improve students' education.

Parents in Fairfax have reportedly been lukewarm to the idea of lengthening Mondays. But parents in Fairfax are like parents everywhere: They are backed against the wall for time. They don't have the lobbying organizations the teachers have. Yet it is parents and their youngsters who stand to benefit the most by having school days that are responsive to the schedules of working parents. They stand to benefit the most by having schools provide the extra instructional time that working parents may not be able to have at home.

What the Fairfax system is contemplating would not please some of the entrenched interests. But the net effect of Spillane's efforts would be to make the schools more responsive to modern families and more responsive to the educational needs of today's youngsters. He deserves the support of parents.