How does smokin pot make you dumb?

i was talking to this guy and i aksed if he blazed and he said yea but i gotta wait for the summer and i asked why he said he has to p math, and before other ppl said that stuff too oh i cant smoke i got a test tomorow. i think wth how would that effect ur schooling? and say u had a test the next day id think if pot made u dumb it doesnt make u less smart over night may take sum time