Take a look at your experiences since kindergarten.
In Kindergarten, 1st and 2nd grade, anyone can really play with anyone on a given day. In fact, boys and girls can be friends with each other and no one really makes comments. Kids of different races can be friends and no one makes comments. There aren't cliques really yet... and if there are, they are usually influenced by parents...
Changes occur throughout the grades 3-8. LOTS of them. Kids say mean things and suddenly, it doesn't just roll off your back anymore, you begin to take some things personally. Your brain is developing even further and throughout puberty, you are especially sensitive. You get to a point at which you just don't want to hurt.
I think by high school students enter cliques for safety. The premise of Anderson's Speak hinges on this thought. The main character was hurt between her 8th grade and freshmen years and is now dealing with that internally. Teenagers long for acceptance because they hope that if they are accepted by the group, no one will hurt them.
Your teacher is doing you a favor by assigning this book... take full advantage of it.