It's 1994 and Erin Gruwell (HILARY SWANK) has just arrived at Long Beach's Wilson High School as an idealistic freshman year teacher. Others there are not as optimistic, such as honors teacher Brian Gelford (JOHN BENJAMIN HICKEY) who thinks the school's recent integration has ruined its reputation and standing, while their boss, Margaret Campbell (IMELDA STAUNTON), doesn't want to see any changes to the current curriculum. That includes not distributing certain textbooks to any classes containing so-called troubled kids for fear of the materials being damaged, lost, or stolen.
Erin's husband, Scott (PATRICK DEMPSEY), is initially supportive of her decision to try to make a difference in the lives of troubled kids including Eva (APRIL HERNANDEZ), Andre (MARIO), Marcus (JASON FINN), Sindy (JACLYN NGAN), Jamal (DEANCE WYATT) and the lone white kid, Ben (HUNTER PARRISH). Yet, her one-time civil rights advocate father, Steve (SCOTT GLENN), thinks she's wasting her time trying to rehabilitate, let alone teach any of them.
Determined to prove all of them wrong, and seizing on some racism-based humor passed around the class, Erin decides to rethink her strategy. That not only includes unconventional class activities, but also getting the students to read "The Diary of Anne Frank" in hopes that it will prove to them that they're not as different from each other as they think and that they're not alone in facing adversity in their lives.
Encouraging the students to write their own journals, Erin tries to make a difference in their lives, all while facing various personal and professional obstacles that stand in her way.