I believe that most people take drugs and other substances to take their mind off of other troubles, or there taking them because its cool.
one thing I suggest is to overcome your shyness and talk to them, ask them strategic questions about how there doing or how there family is . maybe you can become friends with them in the long run.
you could always plan a study group at theirs to see how they live .. maybe there depressed and their trying to be- rid off themselves with drugs .
they might thank you for this too :) take there stress or depression off there shoulders, and have someone to talk to !