Adolescents (and many young[er] adults as well) are in a constant battle with what they perceive as two diametrically opposed desires:
to be their own person;
to be part of a group.
In being part of a group, kids feel pressure to be like everyone else in that group. Many think that they have to sacrifice their identities, or some key aspects of those identities, to gain entry into this safety of numbers.
Self-confidence comes when adolescents realize that they fit into a group not in spite of, but because of, their independence.
And independence is itself a manifestation of who a kid is.
The better "groups", insofar as such "cliques" can be, accept this individualization, however grudgingly; some even celebrate the "craziness" of their members.
Thus it is who a kid is that fortifies the group, not the group that fortifies a kid.
So what is self-confidence anyway?
I don't know.
I do know the operative word in self-confidence:
Self.
Unhyphenated.