As Rand al'Thor has pointed out, the source is a preface that Camus wrote in 1955. In all sources that I have read, this preface is presented as a preface to an "American edition". (For example in L'étranger d'Albert Camus by Bernard Pingaud; Gallimard, 1992; page 55.) The 1962 Pléiade edition of Camus's works, Théâtre Récits Nouvelles, edited by Roger Quilliot, reprints this "Préface à l'édition américaine" on pages 1928-1929 and adds a footnote saying,
Signée du 8 janvier 1955. Publiée par Methuen, Londres, 1958.
(I.e. "Signed/dated 8 January 1955. Published by Methuen, London, 1958.)
Worldcat does not list a school or university edition from 1958, unless the edition by Vintage in New York was such an edition.
Bernard Pingaud's book, which reprints the preface in French, does not mention in which the text was first printed.
Alice Kaplan writes that Camus wrote the preface for "a 1955 American school edition of The Stranger" (Looking for The Stranger; University of Chicago Press, 2018; page 195). WorldCat does indeed list two American editions from 1955: one printed by Prentice-Hall (French text; preface and introduction in English) (an educational publisher) and one by Appelton-Century-Crofts. However, both editions were edited by Germaine Bree and Carlos Lynes, so both editions may contain Camus's preface.
The English version was presumably translated by a native speaker of English, since Camus's English was not even good enough to read Stuart Gilbert's translation of the novel.
The preface has an interesting background. In 1950, Camus's publisher Gallimard had been thinking about publishing the novel in a cheap edition; until then, the novel had only been available in the more expensive cream-coloured edition ("Collection Blanche") with the NRF mark. In July 1950, Camus wrote to Michel Gallimard that he was reticent to see The Stranger leave the selective circle of readers of the Collection Blanche, adding that "Unlike The Plague, it is not a book for everyone" (Alice Kaplan's translation in Looking for The Stranger; University of Chicago Press, 2018; page 195).
One of the reasons for this was the so-called J3 trial that followed a murder by a seventeen-year old schoolboy. (See for example I. - Un fait divers hors série by Jean-Marc Théolleyre in Le Monde, 3 May 1951.) This schoolboy used Camus's novel The Stranger in his defence, and Camus could not claim with certainty that the novel had not inspired the murderer, since he found that "writers bore responsibility for their words" (Alice Kaplan: Looking for The Stranger; University of Chicago Press, 2018; page 194). This is why Camus may have found that he had to explain himself, or at least his novel, in a preface like the one he wrote in 1955.