The Maryland State Board of Education voted yesterday to extend the school year from 180 days to 200, an expensive strategy to improve education that would give Maryland students the longest school year in the nation.

The board voted to phase in the extra days over four years, starting in 1992-93 at an initial cost to the state of $53.3 million. Maryland's 24 school systems would incur a similar expense.

The board's support for the longer school year is, in essence, advice to Gov. William Donald Schaefer and the General Assembly, which would have to approve the extra days and allot the necessary state money. Schaefer said he is interested in the idea, but key legislative leaders questioned the cost.

Under the proposal, school systems could decide where to tack on the extra days, which would add up to more than a thirteenth grade of school by the time a student graduates. Schools could choose to shave days off Christmas and spring vacations, eliminate minor holidays, start earlier in the summer or end later in the spring.

The proposal is the most dramatic and costly part of a 15-point plan to improve elementary and secondary schools introduced last spring by Joseph L. Shilling, Maryland's state school superintendent. The board has recommended to Schaefer and the legislature most other elements of Shilling's plan, which includes preschool classes for disadvantaged children and raising from 16 to 18 the age at which students are permitted to drop out.

Less than two months ago, the board rejected the longer school year, reasoning that it would be expensive and not necessarily the most effective way to improve instruction. But members barely discussed the proposal yesterday before approving it 6 to 3.

"We pretty much knew the majority of the board supported the extension. The real question was whether we could get all the advocates {on the board} there at the same time," said board member Donald P. Hutchinson, a former Baltimore County executive.

Hutchinson said the 20 extra days would help make Maryland high school graduates more competitive with those of other Western nations. He also said the extra learning time is needed because of the "knowledge explosion" over the last generation. "Think about the advances in science. Think about the additional world history," he said.

Yesterday, Schaefer's press secretary, Paul E. Schurick, said the governor is "taken with the notion of a longer school year" but is not prepared to commit himself to the money or legislation that would be required.

Later, Schurick, trying to clarify Schaefer's position, said the governor favored more instruction: "It could be a longer school day or it could be a longer school year." The board yesterday rejected the idea of lengthening the school day.

Legislative leaders questioned the merits and cost of adding days to the school year. Combined with scheduled increases in state education subsidies and the board's $174 million worth of other proposals, the longer year "would eat up all the growth in the budget," said Del. Charles J. Ryan (D-Prince George's), chairman of the House Appropriations Committee.

The creation of a 200- or 220-day school year was a central recommendation of "A Nation at Risk," an influential 1983 report on the condition of U.S. schools. The District and 34 states, including Virginia, have 180-day academic years, according to the Education Commission of the States, a Denver-based group. Only Ohio has a longer year: 182 days.

Several legislatures have debated the idea but rejected it because of the price and parents' objections, said Chris Pipho, the commission's director of state relations.

In 1985, North Carolina agreed to subsidize a 200-day year in two small school districts for three years. One district ended the experiment after three years; the other quit after two, when unhappy parents ousted the local superintendent and school board.

Yesterday, Montgomery County School Superintendent Harry Pitt said, "There is a good argument to support a longer school year for kids." But he added, "The costs would be very high . . . . It is a dilemma for me." Staff writer Richard Tapscott contributed to this report.