The Postoak campus is approximately from the District of Columbia. It serves age 2 to grade 12. The Postoak campus was originally Clagett Farm and later the campus of Harker Preparatory School. The original farmhouse, constructed in 1810, was preserved and renamed Kiplinger House. It is home to the Admission, Advancement, and Business Officers. MacDonald Hall, a multi-purpose theater/assembly/lunch space, features a stage and light/sound booth and seating for approximately 400. In the Main Building, there are classrooms, laboratories, two studios for the visual arts, including ceramics wheels and a kiln, and a darkroom. The two libraries (Lower and Middle/Upper) hold 20,000 volumes and two design labs offer makerspace with tools including a laser cutter, 3D printer, and router.