The Independent has a surprisingly lucid summary of a 25-year longitudinal study of cannabis users from ages 13 to 38. And they found that teen marijuana users dropped their IQs by 8 points:
With no cannabis history, there was a small decline in IQ. However recurrent cannabis use was associated with an 8 point decline in IQ, comparable to that seen in early dementia. Importantly, this decline in IQ was particularly marked when cannabis use began during teenage years. A further concern was that stopping cannabis use did not lead to recovery of the IQ loss. Commentary on the results has ranged from concluding that cannabis is harmful in teenagers but safe in adults, to more cautious notes that adolescent brains appeared more vulnerable to cannabis, without providing carte blanche for longer term safety of cannabis use in adults.
Taking the research at face value, one obvious interpretation is that the developing teenage brain is very susceptible to factors that impair concentration during a critical learning phase; or particularly prone to toxic effect on brain function of chemicals in cannabis.