Most of the readers will think this is obvious but nevertheless:
...this study compared performance of adult marijuana users and non-users...Subjects were given the twelfth grade versions of these tests (Iowa Tests of Educational Development) and other, computerized cognitive tests in successive test sessions. "Heavy" marijuana use (defined by use seven or more times weekly) was associated with deficits in mathematical skills and verbal expression....light" and "intermediate" marijuana use (defined by use one to four and five to six times weekly, respectively) were not associated with deficits. Intermediate use was associated with superior performance in one condition ("fuzzy" concepts) of a Concept Formation test.
From Effects of chronic marijuana use on human cognition, R.I. Block, M.M. Ghoneim, Psychopharmacology (Berl). 1993;110(1-2):219-28.
It seems that if you want to improve your child's score on the Iowa Test, giving him marijuana five days a week is a good idea.