Beginning this fall, Montgomery County ninth-graders can decide for themselves how much time they will spend in gym shorts, or whether to lace up their Adidas at all.

Last week, the Montgomery school board voted to make physical education for ninth-graders an elective, rather than a required course. In doing so, the board also granted a full year's credit -- instead of a half-credit -- toward graduation to all high school students who took two semesters of physical education in the ninth grade.

The change is designed to give ninth-graders more flexibility in picking electives, such as foreign languages, that continue sequentially in grades 10 through 12. It also allows ninth-graders for the first time to earn credit for extra-curricular physical activities.

Under revised graduation requirements, students who already took a year of physical education in the ninth grade will be required to take only one additional semester, or to complete instead two semesters of an approved physical activity outside school hours. Ninth-graders can defer taking any physical education classes at all until their sophomore, junior or senior years.

While the graduation requirement of 1.5 credits in physical education remains the same, some school officials believe that giving ninth-graders the option to delay gym classes cold adversely affect instruction county-wide. The move, they say, could result in a reduction in the number of hours that physical education is offered, constraints on intramural and interscholastic sports, and may cause some coeducational programs to be dropped altogether.

"The worst possible effect," said Ed Masood, director of physical education, "is that we wouldn't need any 'PE' teachers at all in grades 10 through 12."

Already some positions in physical education have been lost to declining enrollment. Physical education as an elective lost additional students when the school day in the county's high schools was cut from seven to six periods.

By allowing ninth-graders a full year's credit in physical education, Masood said, most high school students will need only an aditional half-credit to fulfill the graduation requirement.

Without knowing how many students will opt for gym classes and in which grades, Masood said it is difficult to determine what effects the change will have. Where enrollment does drop, however, resulting cuts in staff could reduce the number of certified teachers available as coaches. And probable staffing reductions as a result of recent budget cuts could force some principals to abandon co-educational gym classes because male and female instructors won't be available during the same hours to supervise locker rooms.

Since physical education instructors are contracted to teach only five periods a day, some schools may have to eliminate one hour of gym from their schedules.

One immediate effect will be an additional administrative burden on junior high schools. Officials will be required to monitor student participation in extra-curricular physical activities. More than 200 county and private agencies offer approved activities ranging from swimming to gymnastics to dancing.