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. To his disappointment, he was relegated to the third-tier football team due to his slender physique. Stewart also made his first onstage appearance at Mercersburg, as Buquet in the play "The Wolves" in 1928. During summer breaks, he returned to Indiana, working first as a brick loader and then as a magician's assistant. Due to scarlet fever that turned into a kidney infection, he had to take time out from school in 1927, which delayed his graduation until 1928. He remained passionate about aviation, with his interest enhanced by Charles Lindbergh's first solo transatlantic flight, but abandoned visions of becoming a pilot when his father steered him towards Princeton.