Here is how and why author Frank O'Connor rejected Sylvia's application to his summer 1953 short-story writing course. O'Connor had first taught the course in summer 1952. He viewed Richard T. Gill as his top student that summer, and chose Gill as his assistant in summer 1953. I quote from the biography Voices: A Life of Frank O'Connor, by James Matthews (Atheneum, 1983):
"At the time O'Connor arrived [at Harvard], Gill had already narrowed down the applicants, among whom was a young woman named Sylvia Plath. O'Connor read her story and rejected her application at once. Gill was curious about the abrupt finality of this judgement and pressed the matter until O'Connor reminded him of the fellow whose madness had nearly fractured the class the year before. To his mind, Sylvia Plath's story suggested a similarly deranged mind. Later that summer his intuitions were confirmed by her breakdown [which was a local and national news story], in which, ironically, he had played some part." [289] . . . .
"Toward the end of the summer [O'Connor] was talking one evening with Dick and Betty [Gill and his wife] about Sylvia Plath's breakdown. Speculating that O'Connor might be harboring some guilt about the incident, Dick broached the subject of neurosis. O'Connor said that he avoided disturbed people because he could not deal with madness. Art to him was discipline . . . "[290]
Sylvia had submitted as her writing sample a typescript of "Sunday at the Mintons,'" published in Mademoiselle in August 1952.
Richard T. Gill (1927-2010) published some short stories, and became a Harvard professor of economics and an opera singer. See Gill's New York Times obituary here. Gill also gave biographer Matthews a detailed account of O'Connor's summer 1952 class (pp. 281-282). [1] Sylvia had met Richard Gill at a party (Journals, Dec. 16, 1958), and didn't like him.
Harvard's rejection letter, never located in any archives, might have told Sylvia she was "too advanced for the class." But schools and writing workshops even today use "you are too advanced" to deny admission to applicants egotistical enough to believe that. In a previous post I addressed the allegation, published in 2013, that Sylvia's mother Aurelia Plath secretly destroyed or hid Harvard's "acceptance letter" to keep Sylvia home that awful summer. Richard Gill's is a first-hand witness's account that has been ignored.
[1] Gill published a very detailed essay about O'Connor's Harvard Summer School courses in Michael/Frank: Studies on Frank O'Connor, ed. Maurice Sheehy (Knopf, 1969). That essay does not mention Plath.