schoolboy error
A very simple, basic, or fundamental mistake or error. Primarily heard in UK. Thanks to a schoolboy error by their opponents' keeper, the football club will advance to the next round of the tourney. The math textbook had to be reprinted after several schoolboy errors were discovered in some of the equations.
schoolboy humor
Immature humor. Guys, enough with the schoolboy humor. Let's try to act like adults, please.
schoolboy scotch
slang A combination of codeine and hard liquor, especially Scotch whiskey. Three people from school have already been hospitalized from OD'ing on that schoolboy scotch!