The graffiti represents rebellion, disrespect, and impatience of teenagers.
The main focus is "I AM IGNORANT". Ironically, the student writing it was ignorant of the implications. Teenagers are so absorbed in their lives, yet they only understand life on a small scale. Ignorance blinds them from life.

On the desk it says "IGNORANCE = BLISS". In other words, let teenagers live in their sheltered microcosm, in "bliss" before being thrown into the real world, into the epitome of true suffering.

The book censoring the "F*ck you" on the wall reads "CATCHER IN THE RYE", the book being alluded.

The blood on the blackboard is the girl's. Her tattoo says "TWLOHA": To Write Love On Her Arms, an organization for helping teens get over depression, suicide, etc.
More irony with the blood (depression) and the pink heart (happiness) she has drawn on the board.