I wouldn't say peer pressure is exclusive to school, but it is one of the major problems that leads kids to drugs, sex, alcohol, smoking, etc. It seems absolutely ridiculous that people would be victims of peer pressure, because saying No! seems so easy. Yet for insecure individuals it's not so easy to accomplish, and in a struggle to 'fit in' and with the 'right crowd' (namely the popular one) in their developing adolescent minds, they are prone to conformity and, worse, to a demoralization of their ability to distinguish right from wrong (for they lack the confidence to trust their own judgment or they lack the confidence to not comply with the requests of others). Hence they participate in wrong, immoral and sometimes illegal behavior, or they change their personality and conform, and end up a mindless bimbo cheerleader/jock. Yes, it is important to define yourself by who you are and not by the way others treat you or by what 'group' you are in, but this isn't an easy task for most people. Just remember that the people who force you to change or do something that you don't want to, aren't the people who you want to please or be friends with, because a true friend accepts you for who you truly are.

Defining yourself as an individual AGAINST the pressure from your peers is one of the essential stresses that forces you to grow up. It's part of biology, and I think we should accept it as reality. You have to find a balance between always being a conforming part of a group and being completely a rebel; it's not easy but it's how you turn into a balanced adult. By the way, joining a group of identical-dressing, identical-thinking and identical-behaving rebels (e.g. a motorcycle club) doesn't count as being a rebel, it counts as conformity.