Montgomery County schools, struggling with an enrollment surge that is expected to continue through the decade, will need an extra $30.8 million next year to maintain the current level of services, Superintendent Wilmer S. Cody said yesterday.

Cody is asking the Board of Education for $434.4 million to operate the system's 152 schools next year, an increase of $34.6 million or 8.6 percent above the fiscal year that ends June 30.

Describing the budget as a "very modest" proposal, Cody said nearly 90 percent of the increase -- $30.8 million -- would be used to maintain services for an anticipated student enrollment of 93,964 this fall, an increase of 2,260 from last fall.

The additional $3.8 million increase Cody is requesting would be used for improvements such as instituting seven-period days at Seneca Valley, Springbrook and Walt Whitman high schools; establishing a new staff training center to assist and coordinate instruction; and buying copiers for each elementary school.

The school board will hold public hearings on the budget Jan. 22, 23 and 24 and must approve it by Feb. 13. The County Council, which levies taxes to support the schools and which historically has tempered school requests, must approve the budget by May 15.

Cody said yesterday the budget was prepared to respond to a dramatic change that has occurred in the past year in the county. Since the early 1970s, student enrollment has declined and school board officials have closed 30 schools in the past three years. But in September that decline appeared to hit bottom when school officials found 800 more students than expected in their classrooms.

The trend is expected to continue, said school analysts who have linked the enrollment jump to higher birthrates and a housing boom, especially in northern Montgomery. By 1990, student enrollment is expected to be 103,000. Enrollment by the end of the century is expected to be significantly higher, Cody said.

The brunt of the budget increase allots $24.6 million -- about 71 percent of the increase -- for employe salaries and fringe benefits. Another $3.8 million would be used to add 159 staff positions to cope with the higher enrollment.

Included in those figures is a 6.5 percent raise for teachers and a 4 percent raise for support staff, both guaranteed in previous labor agreements. Under the agreement, the starting salary for a teacher with a bachelor's degree will be $16,572, up from $15,561 this fall.

Of the operating revenues needed to support the school system next year, the county would provide $363.9 million, a 9 percent increase. The rest of the money would come from state and federal sources.

Cody acknowledged that Montgomery taxpayers would bear most of the cost of the increase. "Because of the county's relatively high wealth, we get very little of the recent, massive increase in state education aid," he said.

Cody's plan to pay for next year's operating costs came a day after County Executive Charles W. Gilchrist recommended that the County Council cut $86 million from a $248 million, six-year building request from the school board. His recommendations were part of his six-year construction budget for the county, which still must be approved by the County Council.

Gilchrist said yesterday that he would not comment on the operating expense request until he had time to review the figures.

But Jacqueline Rogers, Gilchrist's chief budget adviser, said yesterday that the request was about $9 million more than the executive had hoped for. "It portends a difficult budget year in general," Rogers said.

Cody said he hoped that the school district's operating needs would not receive the same treatment from the executive as the capital budget. "I consider this a modest, conservative budget," Cody said of the operating budget. Referring later to the cuts recommended by Gilchrist in the capital budget, Cody said he was "optimistic that wisdom will prevail and those cuts won't go through."

"We may have a AAA bond rating, but we'll have C- schools," Cody said of the executive's concern to preserve the county's coveted bond rating.