 Stewart began attending Mercersburg Academy prep school in fall 1923, because his father did not believe he would be accepted into Princeton if he attended public high school.At Mercersburg, Stewart participated in a variety of extracurricular activities.He was a member of the track team (competing as a high jumper under coach Jimmy Curran), the art editor of the school yearbook, a member of the glee club, and a member of the John Marshall Literary Society.