I feel if I can't beat them, I might as well join them?

Over the past 5 years at school I have made some great friends but over the past year it seems like things have just turned from bad to worst, some of them have even started to smoke. This is not very detailed but my question is if I can't beat my friends into changing them back into who they were, should I just join them on this new exciting life? I've just finished my exams and I feel I have become secluded from not only my best friend but all of them. I've just finished my exams by the way and will be going to college.