THE D.C. school budget, now being reviewed on the Hill, must be adequate to give this city's troubled schools a chance to work properly. But what is a reasonable amount to spend on this result? The school board asked for $265 million for an operating budget in the coming fiscal year. Mayor Barry says that, because of declining enrollment, the schools need only $238 million. The city council set its recommendation between those figures, at $248 million, an increase of about $2 million over this year's school budget.

Into this hodgepodge come Parents United for Full Public School Funding. They argue that even the school board's request is not high enough. Why? Because, they point out, the school system is now running a $10 million deficit. That deficit is due to the rising cost of fuel, last year's early retirement program and pay raises for cafeteria workers. After adding money to next year's budget to prevent another deficit for similar reasons, the parents' group believes, a reasonable budget for the city schools would be about $277 million. They then subtract about $14 millon for cuts in administration and closing some schools, lowering their estimate of what the schools need to about $263 million.

The calculations don't stop there, however. The District's parents note, as parents all over the country will soon be doing, that the proposed Reagan budget cuts will eliminate large shares of federal aid to local schools. The District receives about $13 million from federal programs to help educate poor children and to compensate for military and other government personnel who come into this area and use the public schools. Asking that local funds replace that $13 million in reduced federal aid, the parents' group sets its final estimate of the school system's fiscal needs at about $275 million.

Parents cannot be considered the most objective group on the subject of school funding. But independent budget analysts agree with their basic numbers and facts. The school system is running a deficit this year; the federal government is going to eliminate some aid next year; and with teacher contract negotiations coming up this summer, it is likely that some money will have to be made available for future pay raises.

Last September budget cuts in the schools caused mass confusion as teachers were shifted from one school to another to equalize the effects of layoffs. This school system, which already has its share of headaches, can't afford to have that happen again. Even so, it must make its fair contribution to helping the city out of a very real budget crisis. That is why we think the school system should only get a $19 million increase over last year's budget, instead of the $29 million the parents' group is suggesting. The $10 million difference will have to come from closing as many underused and obsolete schools as possible and from cutting back on administrators. But the public schools are too important and too vulnerable at this time to be denied the balance of the increase.