I think books are challenged due to inappropriate content or themes that might be seen as too mature for audiences of the age range it is targeted towards.
Flight
The book Flight could be one of these challenged books, as its target audience is primarily teens. Flight‘s themes of racism and violence could be seen as too mature for the slightly younger audience it is targeted towards.
Some of the issues that Zits experiences in Flight can relate back to teens today, such as violent urges, poor decision making, helplessness, and self-identity.
Numerous things in Zits’ life could be seen as controversial due to their severity, like being a foster child or problems with alcoholism. These could be unintentional influences toward teens in similar situations to pick up bad habits like drinking or drug use.