The Potomac School in McLean will announce a $3 million to $4 million capital fund drive next month to expand the school through the 12th grade.

The present school includes prekindergarten through ninth grade. Starting in 1987, the ninth and 10th grades will be moved into the new facility, according to school officials. Each succeeding year, an additional grade level will be added to accommodate the rising class until 1989 when the school will be complete through the 12th grade, officials said.

The expansion was prompted by studies showing that parents and students preferred not to have to change schools after ninth grade.

"Studies we have made and consultants we have hired indicate the need for expansion. Many students stay in the area and face the pressure of placing into other schools in the 10th grade," said G. David Hardman Jr., headmaster of The Potomac School.

The school hired the consulting firm Peat, Marwick, Mitchell & Co. to study the area demographics. It also hired Ketchum Inc. to assess the potential funding sources and hired Dulany Bennett to evaluate the area's educational needs. The results confirmed the Board of Trustees' desire to expand the school.

"We are moving very quickly on this. All the studies indicated that this is something that will serve the community and help the school," Hardman said.

The plan calls for constructing a new facility for the ninth through 12th grades and enclosing the present recreational area for use by all the students. Construction should be complete by September 1987.

Founded in Washington in 1904 as a school for girls, The Potomac School moved to the present 70-acre site at 1301 Potomac School Rd., McLean, in 1951. In 1959, the school became coeducational and now has an equal number of boys and girls.

This fall the school will have 540 students, 26 percent from Washington, 16 percent from Maryland and 58 percent from Virginia. The average tuition is $5,400, and this year the school will offer $250,000 in financial aid based on need.

After the expansion, the school will have about 800 students and will add about 18 faculty members.

Potomac stresses a broad educational experience in a traditional approach, taking into consideration the individual student's style and development, according to school officials. "Potomac has a flexible curriculum within high educational standards," Hardman said.

"The area attracts people who are very interested in education. Where there is healthy interest in education, it's good for public and private schools," Hardman said.