Beginning next fall, nearly all ninth graders in Prince George's County will be going to high school, rather than junior high, after the county school board voted Thursday to move up the date when three junior high schools will convert to middle schools.

The board voted to change three junior highs, Kettering, Benjamin Foulois in Morningside and Walker Mill in District Heights, to middle schools this fall rather than the fall of 1985 as scheduled. Eighth graders in those three schools will go to Central and Largo high schools in September.

There will be 28 middle schools this fall, leaving Surrattsville Junior High in Clinton as the county's only junior high. Surrattsville is scheduled to close in June 1984.

The school system began closing down or converting its 32 junior highs to middle schools for seventh and eighth graders beginning in 1980.

School Superintendent Edward J. Feeney pushed the middle-school concept as a way to better utilize school buildings and provide a more structured education for young students free from the influence of the more mature ninth graders.

Some parents from the Kettering area opposed the early conversion of Kettering Junior High and protested during the board meeting that they were notified of the superintendent's intention only last Monday.

"They told us 1984 or '85," said Maryland Secretary of State Lorraine Sheehan, who addressed the board as a Kettering Junior High parent. She added later, "It's just not what we expected. I'm sure it has something to do with the budget."

Sheehan said she worries about sending Kettering's current eighth graders to Largo High School next year. "Nothing is wrong with Largo--if you can handle it," she said.

Other parents who spoke against the Kettering change criticized the middle-school format, which includes extended gymnasium periods on alternate days, shortened class periods to create large blocks of teacher-planning time and a slightly more limited range of courses.

"Most Kettering residents are pleased with the junior high school curriculum," said June Bittinger, who asked the school board to postpone the conversion for a few weeks to give the community a chance to study the changes.

But school board members said that while Kettering residents may have had short notice, the middle-school conversion has been in the works for over two years.

"The issue before us tonight is not if Kettering will become a middle school, but when," said board Chairman Susan Bieniasz, before the unanimous vote to go ahead with the superintendent's plan.