It seems rare that I come across a teen who has not used pot. For the most part, most don't use excessively. When they do use regularly, however, a common scenario occurs: memory starts getting poorer over time, grades begin slipping and academics become less important and much of the teen's socialization centers around pot use. There is something called "amotivational syndrome" that occurs. A person begins to slowly sink into a sense of complacency and apathy, not caring about what used to be important to them. They feel lazy and unmotivated. What makes the problem worse is that the change is so gradual and not noticed. Because of how subtle and incidious the personality change is there is a denial of the changes that have occurred. It's often not until the person stops the use and is abstinent for a while that they will report noticing a difference. Often they notice that they can think better, clearer and they feel mentally sharper. They feel their energy level come back and they feel more alive. All this comes as a surprise because they were oblivious to the downward drift and the toll the pot was having on their body.
I have seen people who appear to have mood symptoms that appear bipolar normalize when they stop they use. Mood swings, anger control and unpredictability all improve. It takes pot a good 30 days to clear out of the system so the results aren't exactly overnight.