[7][4] Endicott's great-grandfather was Salem shipowner and privateer Joseph Peabody, who made a fortune importing pepper from Sumatra, and was one of the wealthiest men in the United States when he died in 1844.

In 1871, when Peabody was thirteen, his parents sent him to England to attend Cheltenham College, a boarding school with a tradition of sending young British men to the military and colonial civil service.

[21] Brooks believed that ETS taught its students to be "eager to train its men to think and reason," and "anxious to blend the most earnest piety with the most active intelligence.

[29] Though Peabody felt unqualified, his stay in Tombstone proved that he could attract donors and manage a congregation, two traits he employed to great effect in his educational career.

[31] It is said that he visited saloons to ask gamblers for donations and "would challenge locals to boxing matches on the condition that if he won, they had to come to church on Sunday,"[29][32] although Peabody dismissed most of these stories as apocryphal.

[34] Regardless of the specifics, Peabody's outgoing manner won admirers among the locals, including Wyatt Earp, whose family donated the altar rail for the new church.

[29] After returning to Massachusetts, Peabody briefly preached at St. Mark's School, his brother George's alma mater,[36] which was looking for a new headmaster at the time.

[38] He initially wanted to put the school in Ipswich, where his forefathers had first landed in America, but eventually chose Groton, where his in-laws lived.

Peabody fashioned the curriculum and lifestyle for boys from upper-class families, whom he wished to steer toward moral leadership, philanthropy, and contributions to the public good.

[44] Although Peabody was ambivalent about his own time at boarding school,[45] he was strongly influenced by Cheltenham's emphasis on public service, declaring that "if some Groton boys do not enter political life and do something for our land it won't be because they have not been urged.

"[50] Peabody had been a good athlete at Cheltenham, and Groton was one of the first American schools to emphasize organized sports as part of the day-to-day curriculum.

Rather, he was theologically moderate and socially puritanical, leading his biographer to write that "[t]heological perplexities and subtleties simply did not affect him ...

"[63] Another writer said that Peabody "stressed social amelioration through Christian principles rather than strict adherence to the fine points of a particular creed.

"[64] Peabody served as the vice president of the Boston Watch and Ward Society, a notoriously censorious organization that gave rise to the phrase "Banned in Boston";[55] when he caught a student reading Esquire magazine, he encouraged the society to target the magazine, either "making it decent or driving it out of existence.

[74][75] He provided important early support to Baguio School in the Philippines, lending it a faculty member and sending his son Malcolm to teach there.

In The Chosen (2005), Jerome Karabel writes that the idea of student merit and achievement that Peabody cultivated at Groton—specifically, the elevation of character and physical accomplishments alongside academic excellence—forms the basis of the modern-day American college admissions system.

[83] Karabel argues that Harvard president A. Lawrence Lowell used extracurricular achievements and unquantifiable character assessments, which typically favored students at upper-class private schools like Groton, to limit the number of Jewish students at Harvard while still maintaining the illusion of merit-based admissions.

"[89] Several of his nonconformist students, like Acheson, Sumner Welles, and Robert McCormick (who did in fact leave Groton), nonetheless went on to distinguished careers.

[57][90] Franklin Roosevelt said of Peabody, "As long as I live his influence will mean more to me than that of any other people next to my father and mother";[53] he invited him to officiate at his wedding and to preach at his inauguration.

"[93] (Ironically, Peabody had voted for Herbert Hoover in the 1932 election; however, he also publicly defended Roosevelt when Groton alumni criticized the New Deal's progressive policies.

[95] On the other hand, Louis Auchincloss harbored a lifelong ambivalence for Peabody,[96] writing that "[t]o my young eyes, and I imagine to most, he seemed to bestride the world like a colossus, but in retrospect I see him more as a David engaged in the seemingly hopeless struggle of preserving some degree of spirituality from the Goliath of materialism that re-invaded the school with each new form of prosperous youngsters.

"[97] A thirteen-year-old Averell Harriman said, “You know he would be an awful bully if he wasn’t such a terrible Christian”; later in life, he told Arthur Schlesinger that "the only recipe for success is to be unhappy at Groton.