The danger and power of high school cliques
The single most important theme in Shattering Glass is the danger and power of high school cliques. Initially, Simon Glass becomes isolated from and ostracized by his peers because he is not a part of a clique (or group with common interests). However, Simon quickly becomes a part of the popular kid clique. And very quickly, Simon becomes popular himself, becoming the odds-on favorite to become the prom king of the school he is attending.
At the end of the day, it was because of high school cliques that Simon was not popular, because of high school cliques that he became popular, and because of high school cliques that other people in his clique murdered him. That is the power, and danger, of the clique: it can cause havoc, death, and destruction in a person's life.
Sadness
Sadness is a pervasive and important theme in Shattering Glass. At the start of the novel, Simon is sad and disengaged from his life and his schooling because he was bullied and ostracized. As a result of being bullied and ostracized, Simon is very sad and depressed. Like Simon, many other kids in the school are sad because of their life circumstances or how they are treated in school.
The corrupting influence of power
Another important theme in the novel is the corrupting influence of power. Rob and his friends have tremendous power and influence in their school. They have the power to make people proverbial kings. They also have the ability to make people social outcasts. Because of this power, Rob and his friends make a king out of Simon Glass, who they turn into the prom king to be. But their ability gets to them and corrupts them, resulting in Rob's group killing Simon.
