Prince George's school officials yesterday unveiled a sweeping plan to close 11 of the county's 41 junior high schools and place most ninth-graders in high schools.

Of the 30 junior highs that remain open, 20 will be converted to "middle" schools handling only the seventh and eighth grades. The remaining 10 will continue as junior high schools.

A ninth-grader attending facilities destined to become a middle school will be sent to one of the county's 19 senior high schools. No senior highs will be closed under the Prince George's plan, which was presented to the school board yesterday.

Last November, the board requested that the superintendent draft the reorganization project after a study predicted that there would be 40,000 costly unfilled seats in county secondary schools by the 1983-84 school year.

Last night the board approved the first phase of the plan, which will go into effect in the fall. Only three schools will be affected. Thomas Johnson Junior High in Lanham will be converted to a pilot middle school. DuVal Senior High in Lanham and Eleanor Roosevelt Senior High in Greenbelt will serve as four-year high schools, taking in ninth-graders from Thomas Jefferson.

If the school board deems the program successful, after the first year, the superintendent then will proceed with the next phase of the program, checking back with the board each year. During phase two, seven junior highs would convert to middle schools, three high schools would get four-year programs and four junior highs would be closed.

If all went according to schedule, all but two of the county's high schools and 10 of the junior high schools would have been affected by the reorganization plan by 1984-85.

According to school officials, the plan was sparked not only by declining enrollment in secondary schools, but also to some extent by ever-tightening fiscal constraints and the reported educational advantages of middle schools.

Reorganization of the secondary school system and conversion to middle schools has been under consideration since 1974, when a task force found middle schools educationally superior to junior schools.

The reasoning was that seventh- and eighth-graders would learn more in middle schools because a special program could be designed for various learning speeds. Moreover, it was argued that ninth-graders would be in a more mature environment and have access to more elections in the high schools.

However, school officials argue that not until this year did it become physically possible because of enrollment declines to begin the reorganization, noting there had not been enough room in the high schools for ninth-graders.

The superintendent's staff prefaced the presentation of the plan yesterday by noting that the number of underutilized secondary schools will have doubled by school year 1984-85.

They noted that while there are seven senior high schools with enrollments of less than 1,500 right now, there would be 13 by the 1984-85 school year. Similarly, there would be 32 junior high schools by then with enrollments of less than 700, in contrast to only 18 now at that level.

The superintendent has argued in the past that when enrollment drops below these levels it is difficult to fund full programs for the school.

"Unless we get more money for staffing, which at this point seems impossible, there's no way we can run comprehensive programs in these underutilized schools," said assistant superintendent Edward Felegy.

Prince George's school officials have estimated that the county would save at least $200,000 for every school closed under the plan.

"We don't want to overplay the financial angle but TRIM [the county's tax-limiting charter amendment] is going to hit us like a cold fist next year," said school spokesman Brian Porter. "Something had to be done to ease the impact."

At present, both Fairfax and Arlington counties send some of their seventh- and eighth-graders to middle schools. Montgomery also has experimented with middle schools and opted that they include sixth-graders as well.

Prince George's junior high schools scheduled to be closed under the reorganization proposal plan are: (1981-82) Belair in Bowie, Frederick Sasscer in Upper Marlboro, Mt. Rainier, Suitland; (1982-83) Rollingcrest in Hyattsville; (1984-85) Beltsville, Bladensburg, Glenridge in Landover Hills, Laurel, Mary Bethune in Chapel Oaks, and Surrattsville in Clinton.