Cliques can be fun and enjoyable, as members are regularly involved in social and recreation activities such as going out together. Clique members can also forge closer friendships, as they get to socialise more often. Some doctors think that cliques can help with the sense of belonging and the social skills of a child as they grow older.
On the other hand, cliques may influence members, through strong peer pressure, to start behavior perceived as bad by others, such as smoking or drug abuse. Problems involving cliques may also be a source of distraction from studies, and this is especially true for outcasts who may feel dejected and tormented. People who are ostracized from cliques can react in a number of ways -- they can lose hope in life, or rally and make a point of becoming better than their tormentor. In the near term, they may decide to start a prank war, look up for inspiration to people like Marvin Heemeyer, or in extreme cases flip out and go Columbine.
http://en.wikipedia.org/wiki/Clique