In 1887, Sherman Day Thacher moved to the Casa de Piedra (CdeP) orange ranch in Nordhoff, California (later renamed to Ojai) with his brother, who was seeking a fresh air cure for his tuberculosis. Thacher's father Thomas was a professor at Yale, and in 1889, Thacher's friend Henry W. Farnam (another Yale professor) asked Thacher to tutor his son for Yale's entrance exams. Thacher agreed to do so, provided that the son move to California for his lessons. According to the Los Angeles Times, Thacher is the oldest private boarding school west of the Mississippi.
At CdeP, Thacher tutored the younger Farnam in both academics and maturity, blending classroom studies with outdoor living and horsemanship. Tuition was $14 a week, "washing excluded." Soon other friends were sending their sons out to California to receive Thacher's instruction, and a school was born. By 1900, Thacher had educated 45 students, and its advertisements counted the presidents of Yale, Harvard, Stanford, and Berkeley as references.