As every schoolboy knows, the standard deviation of a log-normal distribution is directly proportional to its mean. (link)
As every schoolboy knows, Gerry was a statesman of the revolutionary era. (link)
From what little is known of it, they spoke a language unlike any other, unless it be Basque which, as every schoolboy knows, has nothing whatever to do with any other language spoken under the sun. (link)
As every schoolboy knows, Europe's Catholic Right has consisted of reactionaries who began in the service of residual feudal landowners and ended in support of big capital's exploitation and oppression of the masses. (link)
As every schoolboy knows, all Subarus have had all-wheel drive since 1998, so that's a useful part of the package for the Northeast. (link)
As every schoolboy knows, the Langue d'Oeil became the accepted form of French, so the North Men became the Normands. (link)
I believe it was Thomas Macaulay (1800-1859) who discovered the Brilliant Schoolboy, as this quotation shows: "Every schoolboy knows who imprisoned Montezuma, and who strangled Atahualpa" (from his 1840 essay "Lord Clive").
UPDATE: Mark Liberman at Language Log pushes back the use of the "brilliant schoolboy" to 1783, and has even found examples of "every schoolgirl knows." Read the whole post. Mark Goodacre posts about the opposite phenomenon of the Ignorant Schoolboy. Indeed, if you google on the phrase "schoolboy error" or "schoolboy mistake" (as one of Mark's commenters noted) you will get a lot of hits. (Most of them British; draw your own conclusions.) In fact, there is actually a punk band called Schoolboy Error.