Mayor Marion Barry has ordered additional cuts in District agency expenditures to avoid a projected $90 million shortfall this year, including $10 million in reduced spending for the public schools and across-the-board cuts in other departments, city officials said yesterday.

The $10 million reduction in school spending -- from a total budget of $497 million -- could prompt a legal confrontation with the D.C. school board, which met yesterday in an emergency session to consider the mayor's order.

There is some question about whether the mayor has the legal authority to order such reductions in spending by independent agencies, and the school board might sue the city to prevent the reductions, officials said.

An administration official said that Barry's plan has been cleared by the D.C. Corporation Counsel's Office. "The lawyers feel that he is obligated to do this as the city's chief financial officer," this official said.

In a letter circulated to agency managers last Friday, City Administrator Carol B. Thompson said the new spending reductions -- totaling about $45 million of the city's $3 billion annual budget -- are necessary because the D.C. Council refused recently to consider the mayor's revised tax and spending plan.

"He had no alternative but to issue the new levels," Thompson wrote. "Without them, the District could be out of cash before summer's end."

One of the agencies hardest hit by the mayor's latest spending edict is the Department of Human Services, which was ordered to stay within its $695 million budget, even while the department has projected that it will exceed its budget by $40 million. City officials say they're uncertain where to make the cuts in spending for human services.

Even as Barry sought to restrict spending further, the D.C. Council prepared to convene in an emergency session Friday to attempt to block Barry's order to furlough 21,000 city workers, part of his financial rescue package.

The council has approved legislation that would effectively block the furloughs this year, but Barry has indicated he intends to veto it by Friday. By coming back into session, the council could override the veto; otherwise there would be no opportunity to consider the issue before the furloughs take effect next month.

Council Chairman David A. Clarke (D), who formally called the council back from its summer recess, said the "effort is to try to do everything possible legislatively to defend against the furloughs."

Meanwhile, work continued on Capitol Hill on the city's 1991 budget, with the Senate Appropriations subcommittee on the District approving the city's $3.5 billion spending plan for the coming fiscal year.

As part of its action, the subcommittee approved an amendment increasing the penalties -- including mandatory prison time -- for people convicted of selling drugs to minors and using a firearm in the commission of a drug felony.

The panel also approved a boost of about $30 million in the federal funds received by the District, setting aside extra funds to establish a program for babies of drug addicts, to pay lawyers representing children in abuse and neglect cases, and to expand the public schools' early childhood program.

The measure, which would not help with the city's current-year budget problems, would require approval of the full Senate and reconciliation with the House version of the appropriations bill, which contains $6.7 million less in federal funds. The full House is scheduled to consider the city's budget tomorrow.

While Congress appears likely to approve an increase in federal spending for the District, a separate measure that would have boosted federal spending even more significantly apparently has been set aside for the summer, congressional officials said this week.

Under the bill sponsored by Del. Walter E. Fauntroy (D-D.C.) and Rep. Ronald V. Dellums (D-Calif.), chairman of the House District Committee, the federal payment would be fixed by formula at about 21 percent of the city's operating budget, providing a built-in "escalator" for a payment that city officials complain has not kept pace with the rest of the budget. The basic federal payment is now $430 million.

Efforts to forge a compromise between Democrats and Republicans collapsed last week, on the eve of scheduled action by the District Committee, and now officials say the bill will not likely emerge until September at the earliest.

Fauntroy accused Rep. Stan Parris (Va.), ranking Republican on the committee, of reneging on an "understanding" reached at a staff level to a formula that would boost the city's federal payment by as much as $260 million in 1991. He said Parris was pushing a forumla that would result in what he termed a "paltry $34 million" increase.

Parris said there was no agreement. He said any hopes of a compromise were dashed last week after Fauntroy, a candidate for D.C. mayor, publicly called for a "commuter" tax on residents of Virginia and Maryland who work in the District.

Parris said he is dead set against establishing any kind of formula. "That totally eliminates any flexibility or control," he said. "What the D.C. Council could do under those circumstances just boggles the mind."