The District school board took another major step in reducing class sizes yesterday by adding $23 million to the budget proposed by Superintendent Floretta D. McKenzie.

In an emergency meeting called with only a few hours' notice, the board unanimously approved a $482 million spending plan for next year.

The budget, which next goes to the D.C. Council and then to Mayor Marion Barry, rejects McKenzie's call for a "stabilization and maintenance effort," instead continuing the board's push for smaller classes, improved building conditions and strengthened science and summer school programs.

"We're asking for a lot of money, but everything is needed," said board President R. David Hall (Ward 2). "The school system has done its holding steady in recent years. Now maybe it's time for other city agencies to hold."

McKenzie had asked the board to approve a budget that was 6.6 percent higher than the current plan, considerably less than she has fought for in past years. She said her plan was "a direct result of Mayor Barry's urging that all city agencies make significant cuts."

But yesterday, McKenzie threw her support behind the new, more expensive budget that is 12 percent higher than this year's plan.

The additions to McKenzie's budget would greatly expand the number of children in classes that average 20 students rather than 25. The board voted to extend the lower pupil-teacher ratios from the current kindergarten through second grade program to include all third- and fourth-grade classes.

That would require hiring 130 teachers at a cost of $3.3 million.

At McKenzie's urging, the board also agreed to spend an additional $688,000 to expand the smaller classes to fifth- and sixth-grade classes but only in about a sixth of the city's elementary schools, those with unusually high numbers of academically troubled and economically disadvantaged children.

"Going lockstep grade after grade with this program is not helping this city grapple with grave problems of disparities in the quality of life," the superintendent said. Although some board members questioned the fairness of providing smaller classes only in the most needy schools, they went along with McKenzie's plea for extra attention to "children at risk."

The board's additional spending also would extend the smaller classes to all secondary science and social studies classes, at a cost of $4.1 million. And the board added $2.6 million to the budget to hire 101 teachers to bring class sizes closer to the system's goals in kindergarten through second grade, and in English and math classes in secondary schools.

The board also approved an additional $7.5 million in spending for materials and repairs to many of the city's decrepit school buildings. One million dollars of that would go to build science labs in schools that have none.

By making classes smaller, the board has exacerbated the problem of insufficient numbers of classrooms in some schools, especially in neighborhoods with relatively high achievement levels. To ease the problem, the board added almost $2 million to buy and install portable classrooms at those schools.