OED must have received a wide range of quotes for this phrase and cites six of them:
Every schoolboy knows: used to refer to an item of factual information that is supposed to be generally known (sometimes used humorously with reference to obscure or specialized information).
. . 1795 T. Pownall Considerations Scarcity & High Prices of Bread-corn & Bread Pref. p. vi Every school-boy knows that the nut will not shell until it is a brown-sheller.
1840 Macaulay in Edinb. Rev. Jan. 295 Every schoolboy knows who imprisoned Montezuma, and who strangled Atahualpa.
1872 Lancet 27 July 113/2 Every schoolboy knows that if he visit a nest too often, or disturb it in any way, the old birds will desert their little ones.
1900 Cornhill Mag. Sept. 382 The meal which is most characteristic of Yorkshire, as every schoolboy knows, is the high tea.
1963 New Scientist 26 Sept. 658/2 Every schoolboy knows that one sphere can be surrounded by up to twelve others of equal size, so the function g (r) rises steeply to a maximum..then oscillates a few times.
1995 J. M. Kolkey Germany on March ii. 28 As every schoolboy knows, the joint Austro-Prussian military invasion of France was halted short of Paris at a battle that became known as the Miracle of Valmy.’
Nowadays the rise of universal secondary education and college education for the majority there is no GK - just General Ignorance - GI.