For the last three years the D.C. school budget has not kept pace with the city budget as a whole. The overall budget has gone up about 10 percent a year, the school budget less than 6 percent -- not a lot more than required to keep up with inflation. Now come two documents seeking to reverse the pattern. One is school Superintendent Floretta Dukes McKenzie's recommended fiscal 1987 budget, under which spending would rise 11.8 percent in the year beginning next Oct. 1. The other is a report on the fiscal needs of the school system by Parents United for the District of Columbia Public Schools. It says the D.C. system lacks the money to do its job, and offers as standards of comparison the suburban systems in Montgomery and Fairfax counties, which, with fewer problems, are much better off.

The requested city budget is $2.27 billion this fiscal year, of which the school budget is about a sixth -- $360 million. Mrs. McKenzie proposes an increase next year of about $42 million. About half that amount would be for pay raises, inflation adjustments and other steps to maintain current services. The rest would be for improvements. The superintendent, who spent her first years here beefing up the elementary schools, would now add some math and English teachers at the secondary level. Some classes there go as high as 40 students; the recommended number is 26. She would also hire more high school counselors and special education teachers for the handicapped, expand the daylong pre-kindergarten program, buy more classroom computers and devote $3 million to repair of neglected buildings, "the first major increase in this area since 1972." Parents United makes the point that, measured in the traditional way by income levels, need in the city is greater than in the suburbs, yet resources are fewer. The report notes sympathetically that the city system has tended to use its limited extra funds each year for needy students "only at the expense of its other students," but has ended up unable to do enough for either group. At the secondary level, "because of the . . . need for smaller remedial classes . . . basic required classes for average and above-average achievers are oversized. First-year foreign language courses . . . averge about 33 pupils; biology classes average over 34 . . . numerous English classes number more than 30. . . . Even so, many remedial classes still number over 20," the size the Fairfax system aims for.

The typical D.C. school building is 35 to 50 years old, yet the city lags behind the suburbs in its rate of repair. It lags as well, the parents group says indignantly, in per-pupil expenditures on textbooks, library books, supplies, librarians, counselors, coaches, clerks and aides and student health care. A depressing list: the report finds the D.C. staff suffused with "an alarming sense of resignation."

That should not be. The city system is in good hands now, has shown by the rising test scores at the elementary level that it can deliver -- and needs more help. The school board may trim Mrs. McKen the mayor and city council should give the board what it asks, or nearly so. There is no better investment.