As with so many things attributed to Mark Twain, this was not written by Mark Twain. According to https://en.wikiquote.org/wiki/Mark_Twain https://en.wikiquote.

As with so many things attributed to Mark Twain, this was not written by Mark Twain. According to https://en.wikiquote.org/wiki/Mark_Twain https://en.wikiquote.org/wiki/Mark_Twain: Actual source: A letter to The Economist (16 January 1971), written by one M.J. Shields (or M.J. Yilz, by the end of the letter). The letter is quoted in full in one of Willard Espy's Words at Play books. This was a modified version of a piece "Meihem in ce Klasrum", published in the September 1946 issue of Astounding Science Fiction magazine.