The budget that Mayor Walter E. Washington has proposed for the D.C. school system for fiscal 1979 is about $7 million below what school officials say is th absolute minimum cost for which the schools can be run in the fiscal year beginning Oct. 1, 1978.

That minimum, $245 million, "was our bare bones mandatory budget," said deputy school superintendent Edward Winner, who said the figure contained only $12.5 million to fund mandatory increases such as salaries and rising costs for fuel and utilities.

Without that money provided by the city, "our answer has to be it's going to come out of the educational program," said Winner, who added that administrative and personnel costs have already been pared almost as much as they can. The only other area where funds might be found would be in maintenance of school buildings, where money has already been reduced, he said.

The budge that Mayor Washington unveiled only $238.8 million in District funds for the school system, which like other big city systems has faced declining enrollments and increased fiscal pressures in the past few years.

The difference between what school officials said was rock bottom and the $238.8 million was made up of about $3.4 million for expanding special education services, which the city said legally does not have to be done, and fuds for some items that the city agreed must be paid for but which it said could be funded with money taken out of other school programs.

The mayor did not say which programs should be cut but only set the overall budget figure for operating the schools. The school board, which has not yet adopted a budget, makes the choices about where the money will be spent within the system.

The cutback based on special education came in spite of what one school board member said were repeated promises from one of the mayor's closest aides that the city would provide the money for expanded services. Acting on those promises, said board member Carol Schwartz, the board committed itself by adipting new rules to make special education available, over a four-year period, to D.C. residents from 3 to 21 years old.

The city is under a court order to provide free, adequate special education for physically, mentally and emotionally handicapped children from ages 6 to 17. That court order has been the source of some diffculty for city officials, who were found in contempt of court in 1975, when some students were found to be receiving no services.

City compiance with the order has been complicated in the past by disputes over whether the school system or the Department of Human Resources should pay for programs. The mayor's fiscal 1979 budget provides about $1.5 million for improving DHR's special education services. In addition, about $500,000 is provided in the school budget for educational and support services for 90 children at Forest Haven who will attend public schools. Forest Haven is a DHR institution for the mentally retarded.

The fiscal 1979 budget appears to be aimed at reaching compliance with the court order but not going beyond it.

Acting in response to federal legislation encouraging expansion of special education to a larger population, the school board two years ago decided to adopt rules that would gradually increase the population served by the schools from ages 5 to 18 to 3 to 21.

Under the board rules, chilfren aged 4 to 18 were to be covered by fiscal 1979, said Schwartz. The board adopted those rules after city administrator Julian Dugas, who heads a city task force on special education, promised that funds would be available for expansion, she said. Schwartz is also a member of the task force, which was created after city officials were held in contempt for not complying with the court order.

"The great assurances that the money would be forthcoming now seem to be worthless," she said. "I feel very deceived - not me personally, but the board was really assured and encouraged," she said.

Schwartz said she did not know how the board might react, but other sources said that one alternative would be for the board to rescind its rules expanding services. The city budget message, which said that only $700,000 was needed in the fiscal 1970 budget for special education, cited a city corporation counsel's opinion that the city is required by federal legislation only to provide special education for children from 5 to 18 years old.

"We've robbed other programs to pay for special education," said Schwartz, who added that it would be hard to pay for expanded service by taking money out of other programs.

In addition to what school officials called mandatory increases to $12.5 million, Superintendent Vincent Reed had also sent to the mayor a list of $9.2 million worth of programs that might be refunded. The programs, which have been curtailed or ended because of budget pressures, included such items as summer school, which has been cut back severly.

"It's going to have a detrimental effect on the school program," said board chairman Therman Evans. "We've been going through this process now every year I've been on the board. We've submitted only maintenance level budgets, and every year we've been confronted with some kind of a cut or shortfall," he said.

Only days before the mayor's proposed budget was released City Council chairman Sterling Tucker sent Evans a council analysis of public school spending suggesting that 30 schools might be expendable because of declining enrollement. If 30 schools were closed, the system would save $3.3 million, according to the report.

"The report is kind of misleading and some of the assumptions and conclusions are a bit juvenile," said Evans. Because the decline in enrollment is spread across the city, not concentrated neatly in particular schools, the decline of a certain number of pupils does not neatly translate into closing a number of schools equal to serving that number of students.

Evans said the board of education will hear from the pupil on how school system money should be allocated on Sept. 22, when the finance committee meets to consider the budget.