What are the high school clicks that Melinda names? Which ones are important to Speak? Why?

The answer to your question can be found within the first
few pages of the novel.  Melinda has entered the Merryweather High auditorium for what
seems to be the first assembly of the school year.  Melinda immediately notices all of,
what she calls, the high school "clans."

readability="19">

We fall into clans:  Jocks, Country Clubbers,
Idiot Savants, Cheerleaders, Human Waste, Eurotrash, Future Fascists of America, Big
Hair Chix, the Marthas, Suffering Artists, Thespians, Goths, Shredders.  I am clanless.
(4)

Melinda goes on to
explain that her old clan the "Plain Janes" has "splintered," with its members being
absorbed by other clans above.  It is this statement that makes a few of these clans
quite significant (even the one that is now
extinct):

PLAIN JANES:  There
are probably other members of this group, but the main ones were Melinda, Rachel, Ivy,
Nicole, and Jessica (who has since moved to Nevada).  Obviously, their name suggests
that there was nothing incredibly special about them as a group:  average looking girls
doing average things.  That has
changed.

JOCKS:  This clan is
significant because it has absorbed Melinda's "ex-friend" Nicole.  Nicole has always
been athletic and shows lots of promise in the sports arena so she spends the beginning
of school "comparing scars from summer league sports"
(4).

EUROTRASH:  Melinda
doesn't shift Rachel into this group quite yet, but it becomes clearer as the novel
flows.  Rachel begins to be known as Rachel/Rachelle, hanging with all of the foreign
exchange students.  At this first assembly, Melinda something a bit
simpler.

It's
Rachel, surrounded by a bunch of kids wearing clothes that most definitely did not come
from the EastSide Mall.
(4)

MARTHAS: 
This clan is important because it is the precice one that her new "friend" named Heather
tries to join.  They are the ones who always wear matching clothes with accessories and
do volunteer work day and night.

SUFFERING
ARTISTS & THESPIANS:  This is the group that has absorbed Ivy. 
Melinda says that "she has enough personality to travel with two packs" (4).  In my
opinion, it Ivy with her art experience that truly breaks the ice again with Melinda at
the mall, chapters later.