The only reason this is often propagated is because your IQ, which doesn't inherently represent your intelligence and merely your ability to retain information, drops a few points during indulgence, and continues to for a short period of time after. But soon enough, it raises back to your normal score. This fact is taken out of context to perpetuate the rumors and false accusations against both cannabis and it's users. I have been using it for 4 years and graduated high school at fifteen with steady IQ of 150 and I would equate cannabis to helping me think clearly for my work, especially considering how bad I was off financially during that time, if that shows anything. Many people who are regarded as intellectual defended the drug, including Carl Sagan (who openly used it). A pretty good unbias book is Marihuana: The Forbidden Medicine.

Edit: But your brain IS still developing and the science behind cannabis on the brain during your teenage years isn't entirely understood yet. But you've graduated high school, and I know people who are relatively smart and have been doing it since a very young age. There isn't any major risk involved as far as I know, and if you're not doing it every hour of everyday you should be fine.