The Arlington School Board voted last night to close three elementary schools and took another step toward modifying a 1971 desegregation plan under which black students are still bused long distances.

Board members, who took turns giving lengthy explanations for the school closings, voted to close Reed Elementary School at the end of this school year and Claremont and Barrett elementaries at the end of the 1983-84 school year. The board later decided that Barrett will get a reprieve if its enrollment goes up "significantly."

The closings were decided because of declining enrollment--to about 14,000 from 27,000 in the late 1960s--and economic pressures. Some board members said they want to keep better and newer schools as the system becomes smaller.

The vote was not unanimous. Board member Michael Brunner opposed closing Reed and Barrett and sought to postpone the decision to close Claremont until 1984.

Board Chairman Evelyn Reid Syphax voted against closing Reed but supported shutting down the other two.

Brunner, who has announced his intention to seek a seat on the County Board this year, said he felt that Reed Elementary, built in the 1930s, was being closed because of its condition. "I can't recommend or justify closing Reed," he said, citing the school's highly praised instructional program and the fact that 90 percent of its pupils walk to school.

The board's action came before a packed audience in the Arlington Education Center. PTA representatives from each of seven schools considered for closing made last-minute appeals to keep their schools open.

Barrett "is an enigma--all that you've said you want in a school," Barbara Baker, a parent, told the board before the decisions were made. She reminded school officials that the system spent $800,000 in 1975 to renovate the school, which now has a gymnasium and media center.

Instructional "programs can be moved," said board member Claude Hilton, "facilities cannot."

A vote to shorten the bus rides of black students attending schools throughout the county under a highly criticized desegregation plan was postponed by board members, pending an opinion from the school board's attorney on how the plan, approved by a federal court judge 12 years ago, can be amended.

Last night, however, Syphax pushed through a motion to consider sending the black students to either one or all of four schools suggested in a recent staff report. The schools, Barcroft, Abingdon, Randolph and Barrett, are all located in South Arlington. None of the schools requires more than a 12-minute bus ride from the Drew and Hoffman-Boston areas where the children live.

Superintendent Charles E. Nunley had recommended keeping Barrett elementary open. He said the area around it is likely to have significant population growth and cited announcements that more than 900 units in the Buckingham Apartment complex, which feeds children into the school, would be offered as rental units.

Students currently attending Reed will be transferred to McKinley, Nottingham and Tuckahoe schools, according to plans approved as part of the consolidation decisions. Claremont students will attend nearby Abingdon, according to those plans.

Claremont parents had not vigorously opposed closing that school, but last night repeated requests to postpone the shutdown until 1984. PTA President John Lopez said a last-minute informal poll of parents showed 41 percent of them favored the 1984 closing.

The board also voted to keep the H-B Woodlawn alternative school at the Stratford Building, and approved transferring a program for adults seeking high school diplomas from the Clay School to the Hoffman-Boston building. The board postponed decisions on new secondary school attendance boundaries until June 2.

In other business, the school board voted to tell the County Board it favors giving teachers, now scheduled to get 3 percent raises, an additional 0.75 percent across-the-board raise. The County Board recently added $250,000 to the school budget for raises and on Saturday will review the school board's plans for spending it.