That's weird, because I smoke weed everyday and have a 4.0 GPA. I have a vaporizer, so that completely eliminates any risk of lung cancer and it has never gotten me involved in any sketchy situations, if anything it has gotten me out of those sketchy situations. (when you're high you're less aggressive).
The only bad side effects I've noticed is loss of motivation, but if you're a person who has their shit together, that's not a problem as you can do what needs to be done. I have pretty bad anxiety and was supposed to be put on pills with a similar chemical makeup as MDMA but smoking a tiny amount of weed out of a vaporizer calms it down completely.
I'm not telling you kids to go out and smoke weed, but rather to question what you have been taught. In my opinion a lot of psychological problems could be cured with medicinal marijuana but then the drug companies would lose billions of dollars.
Again, I am in no way condoning the use of marijuana - unless your an uptight bitch, then you should definitely smoke some weed.