That wasn't Mark Twain. Since that's "an official website of the United States government", maybe we should email them. (Edit: I emailed. I wonder if they'll reply. Edit 2: that page was eventually taken down.)

The joke originated as "Meihem in Ce Klasrum" [1] in a 1946 issue of Astounding Science Fiction [2] by the satirically named Dolton Edwards, and was reworked into a British variation in a 1971 letter to The Economist [3]. We had a thread about this not too long ago: https://news.ycombinator.com/item?id=23587507

[1] http://www.angelfire.com/va3/timshenk/codes/meihem.html

[2] http://www.isfdb.org/cgi-bin/title.cgi?57114

[3] https://web.archive.org/web/20200807181810/https://lettersof...