The saying that "10% of life is what happens to you, and 90% of life is what you're going to do with or about it" is not a new one, nor is it exclusive to The Misfits by James Howe. It is, however, apt for Bobby, the protagonist of the novel. 

The meaning of the quote is relatively simple and easy to understand. While ten percent of what happens to us in our lives is out of our control (these are the things that happen to us), the other 90 percent is absolutely within our control (what we do with or about those things which we cannot control). This is true for all of us, though of course we often feel as if nothing is in our control.

When someone says something unkind to us, that is beyond our control; however, we do get to decide how we are going to react to the insult. We can choose to be angry, we can choose to ignore it, we can choose to start a fight, or we can choose a hundred other options as a response to this one insult. Whatever we choose also has consequences; while those may not seem to be in our control, it is the choice we made that prompted the reaction. 

In this novel, Bobby is part of a group called the Gang of Five who are all treated poorly by their classmates because they are somehow different. Instead of getting angry, the group decides to fight back by finding a loophole in the school's rules and running for election. Their primary position is that name-calling should be banned, and the reactions to their campaign are generally positive.

While Bobby and his friends did not choose to be insulted, they did get to choose how they will respond. 

“When you're living through it, though, especially when you are twelve and you think the whole world is changing until you realize it isn't the world, it's you, no piece seems little. It's all so big you think it can kill you. But it doesn't. Which is why the story goes on.”