The Prince George's County school board decided last week to develop detailed plans to move all ninth grade students into senior high school, keep seventh and eighth graders in "middle schools" and close several existing junior high schools.

The middle schools plan, as it is called, would be implemented by 1983 if approved by the school board after a public hearing sometime in the next year.

Declining school enrollment, which has led to dozens of schools closing in Prince George's and severe funding problems caused by TRIM, the county's tax-limiting charter amendment, have apparently made the middle school concept very appealing to school officials.

Deputy Superintendent Allan I Chotiner said at the school board meeting last week that educational reasons were also behind the push for a middle school plan.

He said that currently junior high schools tend to be tightly structured because Maryland graduation requirements include ninth graders. The structure, however, is too rigid for most of the younger students, he said.

According to a report written five years ago when the school system first considered the middle school concept, grouping seventh and eighth graders together would allow the development of a less competitive, more flexible program designed to "meet the needs of the student in transition from childhood to adolescence."

"The reason we are considering this is a desire to move the ninth grade kids to high schools, where they can take advantage of the greater number of programs and develop a program to fit the needs of seventh and eighth grade students," said assistant superintendent Edward M. Felegy.

Because enrollment projections show 46,000 costly, unfilled seats by the 1983-84 school year, school officials would like to adopt a plan soon. The plan would include closing several junior high schools, moving ninth graders to underenrolled high schools and consolidating seventh and eighth graders into a fewer number of junior high buildings.

Most school board members last week said they supported the middle school concept. Board member Susan Bieniasz said, "I think we should get going as soon as possible, It's important in relation to our educational needs and our financial situation."