All school and administrative buildings in the D.C. public school system will open when classes resume tomorrow, although areas of asbestos contamination in 121 buildings have been sealed off so they cannot be entered by students and employes, school officials said yesterday.

Andrew Weeks, director of buildings and grounds for D.C. schools, said contamination from crumbling asbestos, a cancer-causing substance, has been corrected in 41 other school and administrative buildings, including 26 where repairs were required before the buildings could be used.

The largest project, on which work is not expected to be completed until late September, is at Johnson Junior High School at Bruce Place and Robinson Street SE. Weeks said repairs already have been made in educational areas at Johnson but that work is continuing in the recreation room of the school. He said that area will remain closed until renovation is completed.

The system's current asbestos abatement program, which has cost $725,000 since April, is mostly a stop-gap effort to cover up crumbling asbestos to prevent minute fibers from becoming airborne. Estimates on the cost of totally removing the asbestos range as high as $50 million.

The D.C. Board of Education is expected to request at least $20 million in its 1986 budget request to remove all asbestos from public school buildings in the District, and the union representing most nonteaching employes pledged yesterday to help press the city and Congress for the funds.

The promise was made during a meeting between representatives of the American Federation of State, County and Municipal Employees Council 20 and School Superintendent Floretta McKenzie, school and union officials said.

Clara Ward, president of Council 20, said the meeting was held primarily to devise a method for employes to report suspected asbestos hazards and to bring union officials up to date on the current renovation work.

Ward said the union has warned its employes to exercise caution in entering buildings where asbestos contamination has not been corrected, and that a committee of union and school officials was named to plan a "hot line" for employes to report possibly hazardous areas.

School spokeswoman Janis Cromer said large, bright yellow signs have been posted in all areas where asbestos contamination has not been corrected, and the areas have been closed and sealed off so they cannot be entered.

"In some cases, the areas are restrooms, and the students will simply be directed to another restroom," Cromer said. "Some stairwells have also been closed because of crumbling asbestos, and in those instances students and employes will be rerouted through the building to another stairwell."

Boiler rooms are the primary areas where asbestos contamination remains, Cromer and Weeks said. They "are not needed until Oct. 15 when the heating season begins," Cromer said. "Anyone who must enter one of these areas will be provided protective clothing and masks."

Officials said all employes will be trained in proper procedures for working in these areas.

Cromer said that as part of the school system's ongoing asbestos education program a letter was sent to all employes last Friday that included a fact sheet about asbestos and outlined work that has been done in the schools.

"Right now we are working with the D.C. Congress of PTAs and Howard University's Cancer Center to prepare a pamphlet about asbestos that will be sent home to every parent during the week of Sept. 10," Cromer said.

She said that records of asbestos inspections are available to the public at each school building.

The inspections were undertaken after the Environmental Protection Agency levied a $24,000 fine against D.C. schools for not complying with federal asbestos regulations. Those rules require schools to sample and analyze areas of suspected contamination and provide that information at the individual school. They do not require asbestos removal or repairs.