That old story that marijuana impairs maths capability until you stop smoking, while alcohol impairs judgement coordination etc.
I wonder if all those science articles were accurate, and whether it applies in this case.
I think your assumption -- that so many 1st to 12th graders in California are doing marijuana to affect the statistics of the state's maths score -- is unbelievably off the mark, that maybe you should see what substances you yourself are being exposed to.
I think a more realistic assumption would involve the parents. Unfortunately, I know a few kids with mental problems from prenatal exposure to pot, in California actually.
And just as a little FYI, it's "cannabis" these days. "Marijuana" has been deemed racist/culturally insensitive [1]/negative [2]/whatever.