The Montgomery County school system, in a gargantuan effort to close more than 30 schools over the next five years, takes its first painful step in that process this month as hundreds of parents prepare to fight to save their children's schools.

Many are battle-scarred veterans who have cahllenged one or more of the 31 school closings that have taken place since 1973. Others are newcomers who believe democratic procedures require and reward their participation.

More than 40 parent groups, representing schools from throughout the county, have sent letters informing the Board of Education that they intend to submit testimony or suggest alternatives to the 700-page document that identifies 34 schools for closure through 1986, redraws school boundaries and reassigns students in the most comprehensive school planning effort the county has ever undertaken.

The decline of the student population from a peak of 126,300 in 1972 to 98,000 this year, coupled with steadily rising costs for heating, cleaning and repairing buildings, have brought pressure from the County Council to close more schools. Shutting an elementary school saves roughly $150,000 a year in operating costs; closing a junior high school saves about $500,000 and a closing a senior high school saves $1 million.

The 15-Year Comprehensive Master Plan for Educational Facilities, as it is called, was more than a year in the making. All through the winter months parents waited uneasily while school planners analyzed enrollment data, the condition of buildings and population projections. In late May, Superintendent Edward Andrews unveiled the plan and a timetable for public response.

Community groups must submit proposals for changes by July 26. Andrews will give the school board his final recommendations in late August. The Board of Education will hold public hearings in the fall and begin voting on the plan in November.

"The timing is not ideal (because of summer vacations) and maybe that's part of their strategy," said Joan Porter, secretary of the PTA at Bradley Elementary -- one of those slated to be closed next year. She laughed apologetically and added, "You get a little paranoid when something so important to your life gets attacked."

There is an atmosphere of anxiety among PTA members as they wait to see whether the school board will follow the suprintendent's recommendations. Already the board has rejected the plan to shut down Brookview Elementary, although the community asked that the school be closed. It was an ominous sign, PTA leaders said. "The board will follow the plan when it's to its advantage and not follow it when it's not," said one.

School board members said they could not close Brookview because it would mean reassigning the students to other schools before it was known whether Northwood High School might eventaully be closed. The board wants to base its decisions on what it calls a "top-down" approach, starting with a decision on the high school, then on the junior highs that feed into it and finally on the elementary schools.

"There are so blasted many things coming at you at once," said board president Carol Wallace. "It's like a puzzle -- each thing hinges on another. That's why I couldn't vote for the Brookview closure.How can you make boundary changes without knowing which high school is being closed? You must start with the high school decisions."

Some parents in the Brookview area charged that the school board did not want to transfer Brookview's pupils to elementary schools farther north, in some cases to schools some board members' children attend.

"I don't think the board wanted to deal with the assignment patterns to reduce the racial disproportion," said William Pepper, vice president of the Brookview PTA. Brookview has one of the largest minority enrollments in the county.

Further complicating the planning process was a resolution, passed at last week's meeting, to change the county's desegregation plan. It would affect 11 elementary schools and require revision of the massive school-closing document.

Currently, the county's desegregation plan groups elementary schools that have large minority enrollments with largely white schools and splits the grade patterns so that, for example, pupils in kindergarten through third grades would attend Takoma Park Elementary School and those in fourth through sixth grades would go to Piney Branch. The resolution directs the superintendent to come up with a plan that would drop these "clusters," as they are called, and require all schools again to house kindergarten through sixth grade. The board will vote on whether to accept the plan as part of its school closing decisions.

The seemingly clear-cut definitions and ranking of closure criteria, backed up by data all laid out for the public to see and comment upon, are quickly being muddied by the proposals for changes. "Communities are frustrated because they don't know what's coming," said Marilyn Praisner, Banneker Junior High PTA president. "The planners' time has not been used wisely."

Even without the mercurial school board's additions and subtractions to the plan, however, individual communities are already challenging the numbers. Beforfe the school board voted to close Argyle and Leland junior highs and Broome Middle School this year (the first closures in the five-year period, made before the master plan was completed), those communities argued that population projections and cost estimates for building renovations were inaccurate.

The Argyle Junior High School community lost its fight against the closing but won an agreement from the school board to keep the building available for two years in case an influx of grade schoolers warranted its reopening.

Northwood High School, slated to be closed in 1984, is represented by a parents' group called Northwood Community Solidarity, backed by $800 raised by the calss of 1981. "NCS is bracing for a battle with the Board of Education by preparing to challenge the plan on several bases," the group has announced.

The statement said the estimate by school planners that $9.4 million would be needed to modernize the building is "so lacking in authenticity as to border on the fraudulent," and that the group's members believe the number of students in the Northwood area justifies keeping the school open. The community organization believes it has a basis for a legal appeal.

The state Board of Education, foreseeing intermindable appeals from parents whose children attend the 85 or more schools throughout the state that are scheduled to be closed in the next five years, decided it would not hear all appeals. To get the ear of the board, parents' groups must show that the local school board "substantially deviated" from its own rules or violated the law, or that the closing would have a "constitutionally impermissible" effect on students.

Of the 12 appeals the state board has heard since 1964, half were from Montgomery County residents. None, however, was successful.

Regardless of the statistical reasons for closing a school, the final argument remains that the community loves its school and wants to keep it. "You tend to be associated for years and years with your school, as you move from kindergarten to high school," said Nicole Oeschger of the Sligo Community Action Committee.