[the_plus_one]

As an American that grew up in a rural area, this definitely isn't something that would've been taught in a public school here. At best, we had typing classes in middle school, and the (quite old) teacher insisted on two spaces after the end of a sentence, something that disappeared after we stopped using typewriters.

[dhosek]

More to the point, the two spaces after the end of a sentence is a consequence of the typographic style at the time that the typewriter was introduced where additional space at the end of a sentence was employed (this is most common in books typeset in the 18th and 19th centuries, although earlier and later examples exist). Despite what people will tell you, it has nothing to do with legibility for monospace printing, and an awful lot with 19th century compositors getting paid by the line and padding their paychecks by increasing spacing wherever possible. William Morris and his followers, direct and indirect, led to a contraction of word spacing and the elimination of the additional space at the end of sentences in emulation of the best printing examples of the 15th and 16th centuries.